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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!

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You don't. It's confusing, but that's not what this means.
VirtualBox uses the Hyper-V API in Windows and the KVM API in Linux VMs. It's for the guest OS rather than the host.
Per VirtualBox documentation:

Exposing a paravirtualization provider to the guest operating system does not rely on the choice of host platforms. ...and...

Oracle VM VirtualBox's implementation currently supports paravirtualized clocks, APIC frequency reporting, guest debugging, guest crash reporting and relaxed timer checks. This provider is recommended for Windows guests.

From: https://www.virtualbox.org/manual/ch10.html#gimproviders

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  • Sorry, I know it's confusing, since there are so many different permutations of options. But my question is not about exposing a paravirtualization provider to the guest at all. Instead I have a Windows 10 host, that has Hyper-V active, in order to provide WSL2 functionality. Therefore the VirtualBox host can only use Hyper-V backend to run it's VM guests. And then for the VM guests I want to enable Hyper-V's nested virtualization feature. That is nested HW accelerated VT-X. Not para-virtualization.
    – user643011
    Jun 24, 2019 at 16:44
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    @essjae Please refer to the VirtualBox 6.0 changelog. VirtualBox can now use the Windows Hypervisor Platform instead of its own virtualization engine.
    – Daniel B
    Jun 24, 2019 at 17:03
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    @DanielB, yes, that's true, but it doesn't work, bug report for this: virtualbox.org/ticket/18387
    – essjae
    Jun 24, 2019 at 19:43
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    Whether it (currently) works or not is not the question here. // Judging from your answer it appears you misinterpreted the question.
    – Daniel B
    Jun 24, 2019 at 19:58
  • Thank you all for the input and the link to the Hyper-V bug report! Very interresting. I would love to get a helpful answer and I am happy to provide more feedback and change the question with more detail. But let's stop down-voting. That's a bit too harsh. Thanks again!
    – user643011
    Jun 25, 2019 at 13:53

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