This is one of those grey areas where you are allowed to do something that is in fact very evil. On non SBS 2008 servers, you are allowed to install the HyperV role on a domain controller. That is, you are allowed by the OS to install the HyperV role on a server that runs a domain controller as host operating system.
Why is this bad? Because of these issues:
HyperV consumes a lot of resources and works best if it is the only role on the operating system. Other roles do not play well in terms of resource scheduling with HyperV.
HyperV only works well if you assign it a dedicated management interface and a dedicated VM interface. Without this you run into strange issues. If you run the HyperV role on the domain controller, you will have a multi homed domain controller and that creates a whole set of other issues.
I have had several HyperV VM's that triggered a fatal error in the HyperV host, whereby all VM's would die, on a domain controller. I have never seen this behaviour on servers with HyperV as the only role on the host operating system.
The right solution is to install Windows 2008 R2 / 2012 on the physical server, install ONLY the HyperV role, then create VM's for your domain controller and other servers. The host OS may or may not be part of the same domain. For small installations I prefer to keep the host OS in a workgroup. This does not scale however.