Hyper-V cannot be installed: A hypervisor is already running.

If you want to add a Hyper-V Role on Windows Server 2012, under VMware ESXI 5.1, you may receive the error message “Hyper-V cannot be installed: A hypervisor is already running.”

To fix this you need to do the following:

1. First, shut down the Windows 2012 Server virtual machine on ESXI 5.1
2. Download and modify the .vmx file associated with your virtual machine, from ESXI server. 
3. Add the following lines to the end of the .vmx file:

·         hypervisor.cpuid.v0 = FALSE

·         mce.enable = TRUE

·         vhv.enable = TRUE

4. Upload the new .vmx file to ESXI Server
5. Restart your virtual machine.

Use the above at your own Risk.