"Citrix will not, however be open-sourcing XenServer.
While Xen, the hypervisor itself, is open source, XenServer,
according to Citrix, contains proprietary code that makes it much
easier to setup and maintain and is a much more polished and
reliable virtualization platform. In the past, XenServer 5 pricing
started at a suggested retail price of $900 per server, regardless
of how many CPUs or sockets were on the system. Starting soon,
XenServer 5 won’t cost users a penny.
"So how does Citrix plan to make money? By offering a new Citrix
virtualization management product line that adds advanced
virtualization management capabilities to both XenServer and
Microsoft's Hyper-V virtualization technology."