Microsoft announced the new four simplified versions of Windows Server 2012. The editions include a Datacenter, Standard, Essentials and Foundation which are for different scenarios. Microsoft with kill the Windows Server Enterprise edition which was available other versions of Windows Server such was Windows Server 2008 R2.
Also interesting is that there is no difference between functionality in the Datacenter and the Standard edition. In former versions of Windows Server Standard edition the Failover Cluster feature and some other parts were missing.
This is a little Overview how you can license Windows Server 2008 R2 in a Hyper-V Environment. One of the biggest advantages of Hyper-V over VMware are the included Guest OS Licenses. For example if you buy a Windows Server 2008 R2 Datacenter license (of each CPU of your physical Server) you can deploy unlimited Windows Server 2008 R2 Datacenter Virtual Machines on this Host.
This Overview should help you understand how this works.