Xen

In 1999, at the Computer Laboratory of the University of Cambridge, Dr. Ian Pratt and his team launched the XenoServers project. Their vision: build an infrastructure of distributed servers across the Internet where individuals or organizations, once authenticated, could purchase computing resources on demand. The idea was not new in principle, but its technical implementation promised to be ambitious. The name XenoServers refers to the Greek xenos, the stranger to whom one offers hospitality without granting blind trust.

The project’s early steps focused on using Java virtual machines to execute client code. This solution ran into a prohibitive constraint that required rewriting all applications in Java. The team then changed course and aimed higher: virtualizing x86 hardware directly to run complete operating systems. At the time, virtualizing the x86 architecture was a technical challenge. Available approaches relied on instruction emulation and binary rewriting, methods that notably degraded performance.

Faced with these limitations, the researchers invented paravirtualization. The principle involves slightly modifying guest operating systems so they collaborate with a hypervisor, while allowing applications to run without modification. Keir Fraser, a doctoral student on the team, took charge of implementing this idea. He developed the core of the hypervisor and adapted Linux 2.2 to this new architecture. The hypervisor received the name Xen, which condenses that of the initial project.

One of Xen’s technical bets lies in its ability to host virtual machines belonging to different clients on the same physical server. Isolation must be total: a client cannot access another’s data or disrupt their operations, and each receives exactly the resources they pay for. The designers like to illustrate this requirement with a compelling example: simultaneously selling services to Coca-Cola and Pepsi while guaranteeing them absolute confidentiality.

The following years saw the project gain momentum. Laboratories like Intel Research and HP Labs provided financial and technical support. In 2003, the first free version of Xen was released under a dual license: GPLv2 for the hypervisor, BSD for components integrated into operating systems. This BSD license choice was not insignificant: it simplified porting to proprietary systems. Microsoft Research became interested in the project, with Paul Barham leading a prototype port of Windows XP.

The publication of the article "Xen and the Art of Virtualization" at the ACM SOSP conference marked the project’s academic consecration. Internet service providers began deploying Xen commercially, exploiting its capabilities for virtualizing computing, networking, and storage.

Christian Limpach contributed in 2004 to porting NetBSD to Xen before joining the Cambridge team. Under Pratt’s leadership, the open source community around Xen expanded. RedHat, SuSE, Sun, IBM, AMD: all adapted their operating systems or ported Xen to their hardware.

The researchers realized that extending the x86 architecture would make virtualization simpler and more efficient, while reducing the amount of privileged code required. A collaboration with Intel led to the development of Xen’s HVM mode, which leverages Intel’s VTx extensions. This mode avoids certain paravirtualized modifications to operating systems, while maintaining paravirtualization where it provides real gains. AMD-V support followed.

The year 2004 also saw the main contributors create XenSource in response to growing demands for consulting services. Funding an open source-based company in the United Kingdom and Europe remained a challenging path, as these business models were not well established there. Ian Pratt reconnected with Simon Crosby, a former university colleague, and went to Silicon Valley where XenSource secured a first financing round of $8 million from KPCB and Sevin Rosen.

XenSource’s first year focused on open source development. The initial strategy? Provide management tools and collaborate with Linux distributions. But when the latter announced their own solutions, this approach showed its limits. At the end of 2005, Peter Levine and Frank Artale joined the company through Accel Ventures. Their commercial vision reoriented XenSource toward direct confrontation with VMware. A strategic partnership with Microsoft in 2006 strengthened the company’s credibility.

In 2007, Citrix acquired XenSource. Disappointed by VMware’s entry into the desktop virtualization market, which directly competed with its application virtualization business, Citrix saw in this acquisition the opportunity to have a Microsoft-compatible platform.

Meanwhile, Amazon built its cloud infrastructure on Xen starting in late 2005. With millions of virtual machines deployed, Amazon is Xen’s largest user, and probably the one deriving the greatest financial benefit from it. Amazon’s ability to virtualize Windows rests on the adoption of Intel platforms equipped with virtualization extensions. Since then, at AWS, Xen has gradually given way to Nitro, the in-house solution.

In September 2022, following Citrix’s acquisition by Vista Equity Partners and Evergreen Coast Capital, XenServer became an independent business unit within the Cloud Software Group. This reorganization opens a new chapter for a technology born in a British university laboratory.

Xen’s success testifies to the role of open source in technological innovation and the capacity of academic projects to produce solutions that leave a lasting mark on the industry.