VirtualBox now supports KVM and Hyper-V as paravirtualization providers

Development
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To be found in VirtualBox VM settings.

In VirtualBox documentation:
https://www.virtualbox.org/manual/ch10.html#gimproviders

(And you can also search that page for other mentions of “paravirtualization”.)

So now in theory, on the Linux platform, one could use the VirtualBox GUI in conjunction with KVM as backend. I recommend against switching VirtualBox paravirtualization providers setting to KVM. It’s unsupported.

Edit:
Edited.

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Using paravirtualization provider Hyper-V on Windows resolves issue Whonix only starts after several attempt - rcpu_preempt self-detected stall on CPU.

This resolved the issue for me. Now documented here:
Windows only - Virtual Machine only starts after several attempts - rcpu_preempt self-detected stall on CPU