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.
| License | License models | Physical | Virtual |
| Windows Server 2008 R2 Foundation | Server License | 1 | 0 |
| Windows Server 2008 R2 Standard | Server + CAL Processor or SAL |
1 | 1 |
| Windows Server 2008 R2 Enterprise | Server + CAL Processor or SAL |
1 | 4 |
| Windows Server 2008 R2 Datacenter | Processor + CAL | 1 | unlimited |
| Windows Server 2008 R2 for Itanium-Based Systems | Processor + CAL | 1 | unlimited |
| Windows Web Server 2008 R2 | Server License | 1 | 0 (or 1) |
| Hyper-V Server 2008 R2 | Free | 1 | 0 |
If you need more infos you can find this here.
