Citrix Xen and Open

three things to consider:

(1) Citrix/XenSource delivers roughly 66% (stats since xen 3.0 release)of the Xen hypervisor code check ins and employs amongst others the project maintainer.

(2) The added value to the hypervisor or engine to stay with that analogy comes through Citrix unique management infrastructure (which is not OSS) as well Citrix partners (HP, Dell, Microsoft, NetApp, Equallogic, Marathon, VMlogix, ...) and ecosystem (Marathon, Vizioncore, Symantec, ...). The management infrastructure is received well even in the linux community (e.g. Linux-Magazin, 03/09, http://www.linux-magazin.de/heft_abo/ausgaben/2009/03). In case you prefer CLI instead of GUI, compare 'xm' and 'xe help -all' commands

(3) The Xen hypervisor is the key to all major products: XenServer, XenClient, XenDesktop and Citrix Computing Cloud (C3, XenServer paired w/NetScaler, WANscaler and Edgeside) - even XenApp, with XenServer being the best platform to harbour the former presentation server product. The largest production clouds are already based on Xen hypervisor. Even Terremark on stage w/ Paul Maritz utilizes xen based EC2 infrastructure (you could see the EC2 reference during their keynote demo).

Back to bullet number #1, Citrix is #1 sponsor of the xen project.