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. Probably not necessarily with the tested and maintained Whonix Libvirt XML Files for KVM ( GitHub - Kicksecure/libvirt-dist: Libvirt XML Files for Derivative Linux Distributions KVM - https:/www.kicksecure.com/wiki/KVM / https://www.whonix.org/wiki/KVM ).
Although I personally like the VirtualBox gui interface a lot more than virt-manager, I would speculate, that HulaHoop (maintainer of KVM Whonix ™ for KVM)) will not be interested in this. This is because reasons for “Why Use KVM Over VirtualBox?” ( Whonix ™ for KVM ) still apply.
I recommend against switching VirtualBox paravirtualization providers setting.
It’s unsupported. ( Frequently Asked Questions - Whonix ™ FAQ )
It would perhaps interfere with the tested VM settings for for VirtualBox ( https://github.com/Whonix/Whonix/blob/master/build-steps.d/2600_create-vbox-vm ) and KVM.
Something that would have to be tested beforehand.
Supporting these use cases would require dedicated maintainers. ( Ways to Contribute )