Like I said earlier – this is still just a “preview” of this feature. Obviously, this feature should not be used in production environments.  Below is a list of known issues:

• Both hypervisors need to be the latest versions of Hyper-V. Other hypervisors will not work. Windows Server 2012R2, or even builds prior to 10565 will not work.
• Once nested virtualization is enabled in a VM, the following features are no longer compatible with that VM. These actions will either fail, or cause the VM not to start:
  • Dynamic memory must be OFF. This will prevent the VM from booting.
  • Runtime memory resize will fail.
  • Applying checkpoints to a running VM will fail.
  • Live migration will fail.
  • Save/restore will fail.
• Once nested virtualization is enabled in a VM, MAC spoofing must be enabled for networking to work in its guests.
• Hosts with Virtualization Based Security (VBS) enabled cannot expose virtualization extensions to guests. You must first disable VBS in order to preview nested virtualization.
• This feature is currently Intel-only. Intel VT-x is required.
• Beware: nested virtualization requires a good amount of memory. I managed to run a VM in a VM with 4 GB of host RAM, but things were tight.