Virtual private server (VPS) is a marketing term used by Internet hosting services to refer to a virtual machine for use exclusively by an individual customer of the service. The term is used to emphasize that the virtual machine, although running in software on the same physical computer as other customers' virtual machines, is functionally equivalent to a separate physical computer, is dedicated to the individual customer's needs, has the privacy of a separate physical computer, and can be configured to run as a server computer (i.e. to run server software). The term Virtual Dedicated Server or VDS is used less often for the same concept.

Each virtual server can run its own full-fledged operating system and can be independently rebooted.

Xen originated as a research project at the University of Cambridge, led by Ian Pratt, senior lecturer at Cambridge and founder of XenSource, Inc. This company now supports the development of the open source project and also sells enterprise versions of the software. The first public release of Xen occurred in 2003. Citrix Systems acquired XenSource, Inc in October 2007 and subsequently renamed Xensource's products under the Citrix brand:
XenExpress became "XenServer Express Edition" and "XenServer OEM Edition" (embedded hypervisor)
XenServer became "XenServer Standard Edition"
XenEnterprise became "XenServer Enterprise Edition"

Subsequently the product lines have been renamed XenServer (Free), Essentials for XenServer Enterprise, and Essentials for XenServer Platinum.

On 22 October 2007, Citrix Systems completed its acquisition of XenSource, and the Xen project moved to 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 AB charter assigns it oversight of the project's code-management procedures, and with development of a new trademark policy for the Xen mark, which Citrix intends to freely license to all vendors and projects that implement the Xen hypervisor; the Xen AB will have sole responsibility for the requirements of licensing.