Xen is an open-source hypervisor that grew out of research at the University of Cambridge Computer Laboratory. In a Xen Project retrospective, founder Ian Pratt recalls that “Xen started life at the University of Cambridge Computer Laboratory, as part of the XenoServers research project,” an effort “to build a public computing infrastructure on the Internet led by Ian Pratt and Keir Fraser.” The same page describes Xen as “the first open source hypervisor for the data center, the very foundation of the cloud as we know it.”
The system was introduced to the research world in the paper “Xen and the Art of Virtualization,” presented by Paul Barham and colleagues at the ACM Symposium on Operating Systems Principles (SOSP) in October 2003. The paper presents Xen as a virtual machine monitor for the x86 architecture that can host multiple commodity operating systems on a single machine while preserving performance and resource isolation.
Xen’s distinguishing technique was paravirtualization: rather than perfectly emulating hardware, Xen presents a slightly modified machine interface, and guest operating systems are ported to run on it. This avoids the costly traps and emulation that full virtualization required on the x86, letting many guests run with close to native performance.
That performance and isolation made Xen attractive for hosting many tenants on shared servers, and it became the engine of much of the early cloud era. The Xen Project, now under the Linux Foundation, frames Xen as having been “the foundation of many of the world’s largest clouds,” a role that grew directly out of the XenoServers vision of renting slices of remote machines.