I did this in a Ubuntu 22.04 guest, but pretty sure the same instructions are valid for Debian:
i got virtio working by adding the following lines to /etc/initramfs-tools/modules
```
virtio_pci
virtio_blk
virtio_net
```
then running `sudo update-initramfs -u`
then rebooting

and pretty sure it's running because my disk IO performance almost doubled, the speed of
dd if=/dev/zero of=testfile bs=10M iflag=fullblock status=progress
went from like 100MB/s to 270MB/s
pretty sure the same would work in Debian. I got my info from
https://wiki.debian.org/DebianKVMGuests
and other places (that link mentions virtio_pci and virtio_blk but not virtio_net for some reason, i got that from somewhere else)
also when i run sudo dmesg | grep -i virtio
i get:
root@test:~# dmesg | grep -i virtio
[ 8.079545] scsi host2: Virtio SCSI HBA
[ 8.129612] virtio_net virtio2 ens18: renamed from eth0