The University of Cambridge Computer Laboratory developed the first versions of Xen.The Xen Project community develops and maintains Xen Project as free and open-source software, subject to the requirements of the GNU General Public License (GPL), version 2.The dom0 domain is typically a version of Linux or BSD.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 para-virtualized 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.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 Xen Advisory Board advises the Xen Project leader and is responsible for the Xen trademark, Citrix uses the Xen brand itself for some proprietary products unrelated to Xen, including Xen App and Xen Desktop.
This is known as hardware-assisted virtualization, however in Xen this is known as hardware virtual machine (HVM).
Xen Project boots from a bootloader such as GNU GRUB, and then usually loads a paravirtualized host operating system into the host domain (dom0).
Xen originated as a research project at the University of Cambridge led by Ian Pratt, a senior lecturer in the Computer Laboratory, and his Ph D student Keir Fraser.
The first public release of Xen was made in 2003, with v1.0 following in 2004.
Soon after, Pratt and Fraser along with other Cambridge alumni including Simon Crosby founded Xen Source Inc. The Open Source Xen Project continued to be supported by Xen Source, then by Citrix following Xen Source's acquisition in October 2007.