Recently I was setting up an environment of two Hyper-V 2012 R2 servers, my plan was to manage these instances of core Hyper-V from a full Windows Server running 2012 R2 to benefit from the GUI Hyper-V Manager. I decided to use my Domain Controller so the first step was to install the Hyper-V server role, when I did this I received the following error "Hyper-V cannot be installed: A hypervisor is already running." this was of course because my Domain Controller is actually a Virtual Machine.
The work around for me in this case was to use PowerShell to install only the Hyper-V Management Tools to allow for remote management. I used the following PowerShell command to find the exact name of the feature I had to install;
To install the management tools use the following command;
After a reboot you should be able to launch the Hyper-V Manager, you can then use the Connect to Server.... button to establish connections to the Hyper-V servers.
If you actually want to run Hyper-V 2012 R2 as a nested hypervisor capable of running nested Virtual Machines, please consult the following blog post http://blog.ryanbetts.co.uk/2014/06/using-vmware-workstation-10-to-run.html you basically have to add hypervisor.cpuid.v0 = "FALSE" and mce.enable = "True" to the Hyper-V VM's VMX file (if you are using VMware Workstation).