I have installed Windows Server 2008 R2 Datacenter x64 as my host (dual boot system), and installed Hyper-V manager (next is Virtual Machine Manager).
Do I need to download and install Hyper-V Server R2? Why would I need this when Win Server has Hyper-V? Is the separate download a newer version?
Hyper-V R2 Server is a standalone hypervisor operating system, analogous to VMWare ESXi.
You do not need to install or download it if you're running a full Windows Server install, you just need to add the Hyper-V role to the server.