Burton Group analyst company did a side by side comparison of VMware's vSphere 4, Citrix's XenServer 5.5, and Microsoft's Hyper-V Server 2008 R2, which is due to ship with its Windows Server OS upgrade in October. Burton Group compiled a list of criteria for running enterprise applications in a virtual environment, including features related to high availability, live migration, memory management, security, networking, storage, licensing and power management. The company admits that its list of features won't be required for all scenarios, but it sees them as a good general guide. It also noted that Microsoft has won some customers for its existing Hyper-V products, especially among smaller businesses and for departmental use.
More info: Computer World