It seems some CPU's will handle HYPER-V in a VM running on a Host where a HYPERVISOR (KVM in this case) is already running.
One needs to make a few mods otherwise booting up the VM running UNDER the HYPER-V (the 2nd level VM) will freeze.
I'm currently playing with this -- it looks like some modules from QEMU need to be updated -- and one needs to use IOMMU to get any sort of decent I/O throughput - even on SSD's.
I'll post back when I've got some better results -- note I haven't a CLUE about AMD CPU's . I think also getting USB and SOUND on to the second level VM could be a problem. PCI passthru is pretty easy from the KVM Host to the Windows VM (ist level) but passing that to the second level VM is proving somewhat problematical.
(HOST --CENTOS 7 with KVM, ist level VM W10 pro with HYPER-V enabled, second level VM W7 ultimate -- all x-64 machines).
I believe also performance can be improved by switching from Full virtualisation to "Para virtualision" on the guests -- easily enough done with settings in KVM - but then passthru gets a bit ????.
Anyway I've got some time to play with all this --when I'm done I'll post a guide. It should work also for most other common Linux distros capable of running KVM.
Tip - don't use the libvirt manager GUI as a console for your VM's as once in Full screen you can't always get back to Windowed mode -- might be my mouse etc. Use RDP / VNC instead -- no problem connecting to VM's even if you are running the VM's on your local machine.