From the Citrix blog :
If you’re reading this, you already know the news: XenServer, our enterprise virtual infrastructure platform is now free (including resource pooling and live relo), and we have announced Citrix Essentials for XenServer, and Citrix Essentials for Hyper-V as our virtualization management portfolio that offers a rich set of automated functions that drive the compatible virtualization layers beneath – the free Hyper-V hypervisor from Microsoft, and the free XenServer Enterprise virtual infrastructure platform from Citrix. Finally, and most importanly, we announced a powerful go-to-market roadmap with Microsoft.
XenMotion will let you move VMs between boxes without downtime…for free. Nice!