Workload Balancing has been re-introduced within XenServer 6.5 Enterprise Edition: Workload Balancing (WLB) automates the process of moving Virtual Machines between hosts to evenly spread Network, CPU, and Disk loads to maximize throughput. WLB keeps a history of the usage of CPU, Disk, and Network for all VMs in the pool so it can predict where workloads can be best located.
XenServer v6.2.0 included additional monitoring features for customers who wish to capture performance and throughput data. Additional 3rd party products which support workload balancing functionality are available from vendors such as VMTurbo, Lanamark, CA Technologies, Goliath, and eG Innovations.
Details here: http://bit.ly/1DLF6Y9
Citrix, XenServer/6.5, Commercial Edition
Automated Live Migration, VM Mobility, VM Mobility and HA