What is Xen?
Xen.org, home of the Xen hypervisor, the powerful open source industry standard for virtualization. It is a native (bare-metal) hypervisor providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently.
Comments and Reviews
Said about Xen as an alternative
It is an opensource and it is better solution for small sized companies or for new startups. I would recommend the entrepreneurs definitely
Amazon and zillions of enterprise shops use it where VMware would be too expensive.
XenServer is based off of Xen.