Q: How many newer versions of Windows can I run on an older Hyper-V server?

A: Microsoft has an n+1 policy related to running newer operating system versions on Hyper-V. For example, on a Windows Server 2008 R2 Hyper-V server, you can run Windows Server 2012 (but not Server 2012 R2); on a Windows Server 2012 Hyper-V server, you can run Windows Server 2012 R2. This isn't to say that you absolutely can't run a newer operating system; however, doing so isn't supported and might not work properly.