Xen

XenServerXenSourceXen Cloud PlatformCitrix XenServerDom0Citrix XenDomUXen HVMXen Loadable ModuleXen Servers
Xen Project (pronounced ) is a type-1 hypervisor, providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently.wikipedia
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Intel

Intel CorporationIntel Corp.Intel Inside
developed by the University of Cambridge and is now being developed by the Linux Foundation with support from Intel. This move had started some time previously, and made public the existence of the Xen Project Advisory Board (Xen AB), which had members from Citrix, IBM, Intel, Hewlett-Packard, Novell, Red Hat, Sun Microsystems and Oracle.
The Open Source Technology Center at Intel hosts PowerTOP and LatencyTOP, and supports other open-source projects such as Wayland, Mesa3D, Intel Array Building Blocks, Threading Building Blocks (TBB), and Xen.

Hypervisor

hypervisorsvirtual machine monitorhost machine
Xen Project (pronounced ) is a type-1 hypervisor, providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently.
Similar trends have occurred with x86/x86-64 server platforms, where open-source projects such as Xen have led virtualization efforts.

Ian Pratt (computer scientist)

Ian Pratt
Xen originated as a research project at the University of Cambridge led by Ian Pratt, a senior lecturer in the Computer Laboratory, and his PhD student Keir Fraser.
He was the chief architect of the open-source Xen project, and chairman of Xen.org.

Department of Computer Science and Technology, University of Cambridge

University of Cambridge Computer LaboratoryComputer LaboratoryUniversity of Cambridge Mathematical Laboratory
Xen originated as a research project at the University of Cambridge led by Ian Pratt, a senior lecturer in the Computer Laboratory, and his PhD student Keir Fraser. The University of Cambridge Computer Laboratory developed the first versions of Xen.
Members have been involved in the creation of many successful UK IT companies such as Acorn, ARM, nCipher and XenSource.

NetBSD

BSDCryptographic Device DriverNetBSD Foundation
The dom0 domain is typically a version of Linux or BSD.
The Xen virtual-machine monitor has been supported in NetBSD since release 3.0.

Paravirtualization

paravirtualizedvirtualizationpara-virtualized
User domains may either be traditional operating systems, such as Microsoft Windows under which privileged instructions are provided by hardware virtualization instructions (if the host processor supports x86 virtualization, e.g., Intel VT-x and AMD-V), or paravirtualized operating systems whereby the operating system is aware that it is running inside a virtual machine, and so makes hypercalls directly, rather than issuing privileged instructions.
For example, the Xen Windows GPLPV project provides a kit of paravirtualization-aware device drivers, licensed under the terms of the GPL, that are intended to be installed into a Microsoft Windows virtual guest running on the Xen hypervisor.

X86 virtualization

Intel VT-xAMD-VVT-x
User domains may either be traditional operating systems, such as Microsoft Windows under which privileged instructions are provided by hardware virtualization instructions (if the host processor supports x86 virtualization, e.g., Intel VT-x and AMD-V), or paravirtualized operating systems whereby the operating system is aware that it is running inside a virtual machine, and so makes hypercalls directly, rather than issuing privileged instructions.
A different route was taken by other systems like Denali, L4, and Xen, known as paravirtualization, which involves porting operating systems to run on the resulting virtual machine, which does not implement the parts of the actual x86 instruction set that are hard to virtualize.

Amazon Elastic Compute Cloud

Amazon EC2EC2Elastic Compute Cloud
Amazon EC2 (since August 2006), IBM SoftLayer, Liquid Web, Fujitsu Global Cloud Platform, Linode, OrionVM and Rackspace Cloud use Xen as the primary VM hypervisor for their product offerings.
Initially, EC2 used Xen virtualization exclusively.

Open vSwitch

vSwitch
Open vSwitch is the default network switch in the XenServer virtualization platform since its version 6.0, and in the Xen Cloud Platform via its XAPI management toolstack.

Linux Foundation

The Linux FoundationPatent CommonsEmbedded Linux Conference
developed by the University of Cambridge and is now being developed by the Linux Foundation with support from Intel.
The Xen Project team is a global open source community that develops the Xen Hypervisor, contributes to the Linux PVOPS framework, the Xen® Cloud Platform and Xen® ARM.

Simon Crosby

Soon after, Pratt and Fraser along with other Cambridge alumni including Simon Crosby founded XenSource Inc. to turn Xen into a competitive enterprise product.
Prior to XenSource, he was a Principal Engineer at Intel.

OrionVM

Amazon EC2 (since August 2006), IBM SoftLayer, Liquid Web, Fujitsu Global Cloud Platform, Linode, OrionVM and Rackspace Cloud use Xen as the primary VM hypervisor for their product offerings.
OrionVM uses the Xen hypervisor to virtualise multiple machines (referred to as "instances") on the same hardware.

Hardware-assisted virtualization

virtualizationaccelerateshardware assists
This is known as hardware-assisted virtualization, however in Xen this is known as hardware virtual machine (HVM).
Hardware-assisted virtualization is also known as accelerated virtualization; Xen calls it hardware virtual machine (HVM), and Virtual Iron calls it native virtualization.

Linode

Linode,
Amazon EC2 (since August 2006), IBM SoftLayer, Liquid Web, Fujitsu Global Cloud Platform, Linode, OrionVM and Rackspace Cloud use Xen as the primary VM hypervisor for their product offerings.
Linode transitioned from UML to Xen virtualization in March 2008, and then to KVM in mid-2015.

Rackspace Cloud

RackspaceRackspace Cloud FilesMosso (Rackspace Cloud)
Amazon EC2 (since August 2006), IBM SoftLayer, Liquid Web, Fujitsu Global Cloud Platform, Linode, OrionVM and Rackspace Cloud use Xen as the primary VM hypervisor for their product offerings.
The "cloud servers" are virtual machines running on the Xen hypervisor for Linux-based instances, and Citrix XenServer for Windows and Linux instances.

Novell

Novell, Inc.Novell Inc.PGSoft
This move had started some time previously, and made public the existence of the Xen Project Advisory Board (Xen AB), which had members from Citrix, IBM, Intel, Hewlett-Packard, Novell, Red Hat, Sun Microsystems and Oracle.
SUSE Linux Enterprise Server was the first enterprise class Linux server to offer virtualization based on the Xen hypervisor.

Citrix Workspace

XenDesktop
Citrix uses the Xen brand itself for some proprietary products unrelated to Xen, including XenApp and XenDesktop.
The virtualization technology that led to XenDesktop was first developed in 2000 through an open-source hypervisor research project led by Ian Pratt at the University of Cambridge called Xen Project for x86.

Protection ring

kernel modering 0kernel-mode
On 32-bit x86, the Xen host kernel code runs in Ring 0, while the hosted domains run in Ring 1 (kernel) and Ring 3 (applications).
A renewed interest in this design structure came with the proliferation of the Xen VMM software, ongoing discussion on monolithic vs. micro-kernels (particularly in Usenet newsgroups and Web forums), Microsoft's Ring-1 design structure as part of their NGSCB initiative, and hypervisors embedded in firmware such as Intel VT-x (formerly Vanderpool).

Qubes OS

QubesQubesOS
Virtualization is performed by Xen, and user environments can be based on Fedora, Debian, Whonix, and Microsoft Windows, among other operating systems.

Oracle VM Server for x86

Oracle VM
Oracle VM Server for x86 incorporates the free and open-source Xen hypervisor technology, supports Windows, Linux, and Solaris guests and includes an integrated Web based management console.

QEMU

KQEMUqemu-irixQuick Emulator
Xen HVM has device emulation based on the QEMU project to provide I/O virtualization to the virtual machines.
Xen, a virtual machine monitor, can run in HVM (hardware virtual machine) mode, using Intel VT-x or AMD-V hardware x86 virtualization extensions and ARM Cortex-A7 and Cortex-A15 virtualization extension.

Citrix Systems

CitrixCitrix Systems, Inc.Citrix XenDesktop
This move had started some time previously, and made public the existence of the Xen Project Advisory Board (Xen AB), which had members from Citrix, IBM, Intel, Hewlett-Packard, Novell, Red Hat, Sun Microsystems and Oracle. On October 22, 2007, Citrix Systems completed its acquisition of XenSource, and the Xen Project moved to the xen.org domain.
Citrix gave Cloudstack to the Apache Foundation in 2012 and Xen hypervisor to the Linux Foundation in 2013.

Sun xVM

xVM
The Sun xVM hypervisor was a component of Solaris based on work that was being done in the OpenSolaris Xen community.

HyperVM

HyperVM was a multi-tiered, multi-server, multi-virtualization software product allowing a Virtual private server (VPS) vendor to provision, manage and delegate Xen- or OpenVZ-based virtual private servers.