Virtualbox has its own nested virtualization - currently (version 6.0 2019) only for AMD based virtualization.
However when Hyper-V is enabled, it can't use it's own virtualization, because Hyper-V takes exclusive access to VT-x/AMD-V so it uses Hyper-V as a fallback to start the virtual machine, instead of it's own.
Hyper-V does support nested virtualization as well. How do I direct Virtualbox to start the virtual machine with Hyper-V's nested virtualization enabled?
[Edit]: Notice, I am not asking about exposing a para-virtualization interface to the guest, I am asking about nested VT-x/AMD-V virtualization. Thank you!