Hypervisors are free now, VMware needs to move on
I don't think the plan is for Citrix to make money selling the Xen Hypervisor. Every day Intel, AMD, and others are working to put more of the software virtualization layers functionality into their hardware, as APIs and standards become more solid the hypervisor itself will become invisible in the same way that DOS memory managers and other OS plumbing did 15 years ago.
When the PCI bus can be virtualized safely in hardware I expect the hypervisor software layer to vanish entirely.
Citrix has done what Citrix always does, give some of their core technology to Microsoft for inclusion in their OS (was Terminal Services then, is now the core of XenServer upon which Hyper-Free relies). Citrix will then use this to seed the market for their pay-for management tools and enterprise functionality.