After provision a virtual machine running Windows Server 2012 R2 and we try to install Hyper-V roles, we encountered the following error message
" Hyper-V cannot be installed. A hypervisor is already running".
1. Turn off virtual machine
2. Go to VM Settings
3. Change Guest OS version to Hyper-V (unsupported)
4. Change Processor settings by select "Virtualize Intel VT-x/EPT or AMD-V/RVI and Virtualize CPU Performance counters.
Now it is working fine.