Microsoft Windows Hyper-V Server 2008 R2 is a free version much like Windows Server 2008 R2 Core, except it comes with the Hyper-V role preconfigured.
The only thing you will have to take in consideration before installing such a server is that Hyper-V Server 2008 R2 only comes with a PowerShell/CMD-prompt. So to configure such a server you should learn some Windows commands and PowerShell cmdlets.