Serial console works for Whonix KVM.
Package serial-console-enable might help / simplify setting up the kernel boot parameters inside the VM.
sudo apt-get install serial-console-enable
Serial console may be possible with VirtualBox too. See:
I didn’t manage to set it up yet.
Note of carefulness during experimentation: trying to set it up might brick that VM that you tried to set it up with. Undoing the VirtualBox changes might not actually undo these changes and that VM may no longer boot. I don’t know yet.