One of the things I love about Xen is that it’s been open since its beginnings. Xen came out of the XenoServers research project at the University of Cambridge Computer Laboratory. This project was conceived in the late 1990s to investigate how to build a public infrastructure for wide-area distributed computing, in effect a forerunner of what we call cloud computing today. Early XenoServers work required a way to run multiple workloads on a single computer with strong isolation properties. Although there was a commercial product available at that time that had some suitable properties its cost would have been prohibitive for a global scale research project and the closed nature of the product would have hampered progress and limited flexibility. Instead in 2002 Keir Fraser, then a PhD student of Ian Pratt at Cambridge, created “xenomon” (meaning the “XenoServers monitor”) to enable multiple Linux instances to run in an isolated manner on the same hardware. The Linux instances, or guests, were modified to work on this monitor – a technique now referred to as paravirtualization. Xenomon borrowed heavily from Linux and therefore the project used a GPL license from the earliest days.

XenoServers itself was a large, collaborative research project which meant that xenomon, which soon got shortened to “Xen”, soon started to become a distributed project involving multiple research organizations including university departments and industrial research labs. Largely driven by the 2003 research paper “Xen and the Art of Virtualization” (PDF) Xen took on a life of its own as an open-source project and collaborators could see the value of having an open-source hypervisor to provide an open alternative to proprietary products and create an environment for innovation. The project attracted collaborators from silicon vendors, hardware OEMs, operating system developers and vendors and many other organizations and individuals. In many cases commercial competitors were working side by side on Xen.

©2013 Xen Project, A Linux Foundation Collaborative Project. Used with permission.

In January 2005 Pratt, Fraser and other Xen leaders founded a start-up to provide commercial services and products around Xen. XenSource Inc. continued to lead the Xen open-source project with the company funding a dedicated team to work on it. In 2006 XenSource released its first generally available product “XenEnterprise 3.0” (based on the Xen 3.0 release). The XenEnterprise product line, since renamed to “XenServer”, continued to evolve through Citrix’s acquisition of XenSource in 2007 and today provides enterprise-class, open-source virtualization for cloud, VDI, enterprise, SMB and other users. XenServer is a distribution of Xen and other open-source components built into a coherent system giving users high quality virtualization in an easily consumable form.

Although XenSource and Citrix have led the Xen project they are not the only organizations to build products and services based on it. Well known examples of others include Oracle VM, SuSE Linux Enterprise Server and Amazon Web Services. It was this broad community that led to Citrix creating “” as a home for the project giving some clarity about the boundary between Citrix’s commercial operations and the open-source project. In April 2013 Xen moved to a new home as a Linux Foundation Collaborative Project.

©2013 Xen Project, A Linux Foundation Collaborative Project. Used with permission.

This move reflects the importance of Xen to multiple vendors, service providers, organizations and individuals and provides a truly neutral home which will help ensure Xen continues to evolve to meet the changing needs of server, client and device virtualization. Citrix continues to fund a dedicated team to work within the Xen Project performing software development, being maintainers and providing community services such as automated testing, release management, and so on.

Xen is coming up to its 4.3 release which adds support for ARM’s new virtualization extensions, preparing the way for Xen-based ARM microserver virtualization. The release also includes a number of performance, security and functionality enhancements and is the first release which will use the upstream version of the qemu system emulator by default – this will mean that both major open-source hypervisors will share this common component and collaboration around it will benefit a large number of users.

As chair of the Advisory Board for the Xen Project I am really pleased to see Xen in a truly independent home where individuals and organizations can work together for mutual and shared benefit. As a Citrix employee I am excited about the future of our Xen based products which will benefit from the freedom of collaboration the new Linux Foundation home will give Xen. In this fast moving world we truly are better together.

I encourage you to go and take a look at the Xen Project website ( to see how the project is evolving. To find out more about the history of Xen check out our video at