VMware isn’t the only one anymore doing that magical live migration and Citrix wants everyone to know about the ins and outs:

XenMotion is a feature of Citrix XenServer Enterprise that gives an administrator the ability to move a running virtual machine from one XenServer to another. Virtual machines can be moved from server to server without service interruption for zero-downtime server maintenance. Administrators can move running application work loads to take advantage of available compute power.

Read more at the source.