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How this differs from other free hypervisors is that a number of enterprise features have been included that otherwise would have cost $$.
This includes,
Centralised management
Live migration (not even available with Hyper-V, scheduled for Windows 2008 R2)
Resource scheduling
64bit Hypervisor (ESX is 32bit for example)
Bare Metal Hypervisor
Integrated storage management
(Note High availability is missing from this release, you need XenServer Essentials for that)
XenServer essentials is now the commercial version of XenServer which essentially is a management and feature extension for Xen and Hyper-V.
http://citrix.com/English/ps2/products/product.asp?contentID=683148
Full facts and figures for XenServer 5 can be found here
http://www.xenserver5.com/
For those of you thinking about testing or trailing XenServer 5 it installs in VMware Workstation, however as VMware Workstation does not pass Intel or AMD virtualisation extensions through its emulated BIOS you can not test Windows VMs (unless someone knows something I do not?!) as Xen requires these to virtualise Windows.
Blog, Citrix, Windows
virtualization, xenserver