From Xen

XenExpress was a custom Linux distro that actually uses CentOS as a base. The XenSource folks take CentOS, replace the stock kernel with one with Xen support, strip down the install package set to a fairly barebones level (approximately 190 rpms) and then add in a few custom Xen server components.

XenExpress was rebranded to XenServer Free edition, following the Citrix acquisition of Xensource.

Also See