Citrix has recently joined the Linux Foundation, and there is a report (which they seem to have endorsed) that they plan to open source XenServer. That’s not Xen, it’s XenServer – not the kernel, the product, the thing you stick on your server instead of ESXi, or sometimes vSphere.
It is entirely possible that Citrix’s lawyers have noticed that XenServer was so infected with GPL code that it was already Open Source anyway.