Distro morphing Debian into Whonix is unsupported.
Fortunately, in this use case I don’t see why you would need to resort to distro morphing. PVH vs HVM is just a dom0 setting that you can flip.
Please exercise this with Debian first to avoid asking questions unspecific to Whonix.
You can most likely start with a Debian Template. I see no reason to go through an ISO installation procedure unless you have a specific reason for this (and “HVM” is probably an invalid reason). I am mentioning this since it might be the case you’re making a lot of assumptions of complex steps to take which are unfounded.
If you don’t use distribution morphing, which I don’t see why you would need that, then no… This one I don’t see how that would happen if the VM is still based on Whonix.
Whonix doesn’t make a lot assumptions about the virtualizer. As long as Qubes works as expected so will Whonix.
Whonix source code doesn’t have any concept of HVM vs not HVM. It does not try to determine this or take any specific action based on it.
This question is unspecific to Whonix.
I suggest to rephrase it.
If I use a Debian HVM, can it decrease my security?
Then redirect it to Qubes as per:
Self Support First Policy for Whonix / https://forums.whonix.org/t/what-to-post-in-this-qubes-whonix-forum-and-what-not