Xen is a popular VPS platform. Created in 2003, at the University of Cambridge, Xen achived popularity quickly due to the fact that it was Open Source. Where a VPS host might have to pay a significant amount for competing VPS platforms, a host-on-a-budget could offer Xen hosting without spending as much to set up their business. Initial versions required specially patched kernels, which made supporting Xen sometimes more difficult than its pricier competition. Recent versions can take advantage of special virtualization modes in recent CPUs, allowing for a wider array of operating systems to be usable with the Xen platform.
Xen's virtualization technique is known as paravirtualization. According to the creators of Xen, its use allows the creation of a very tiny hypervisor, resulting in system virtualization overhead 0.1% and 3.5% in benchmark testing.
Xen requires hosts to have as much RAM and disk space as their VPSes could possibly every use. This prevents them from overselling, where they assume that not every user will be using all of their disk and ram at the same time. Competing products allow overselling, which generally allows the host to make more profit on a given server.