If you want to enable Windows 8.1 Preview Hyper-V feature, and your copy of Windows 8.1 is running as a virtual machine in VMware Workstation 9, you’ll find the Hyper-V Platform option is greyed out, and see the message:
Hyper-V cannot be installed: A hypervisor is already running.
To resolve this and allow Hyper-V to run in a VM environment. You need to make two configuration changes.
The first is enable VT-x/EPT or AMD-V/RVI in the Virtual machine settings. To do this shutdown the VM, edit the virtual machine settings, select Processors and put a tick in the box as shown below, and click OK.
The second configuration change is to the virtual machine’s vmx file itself. Again, with the virtual machine shutdown, browse to the location of its vmx file.
Open the file with Notepad, and then add the following line:
hypervisor.cpuid.v0 = “FALSE”
Save the file and start the Windows 8.1 VM. You’ll now have the option to add the Hyper-V Platform.
There is a good article over at VMware Communities about running nested VMs.