This declares a strong, but not absolute, dependency.
The Recommends field should list packages that would be found together with this one in all but unusual installations.
Suggests
This is used to declare that one package may be more useful with one or more others. Using this field tells the packaging system and the user that the listed packages are related to this one and can perhaps enhance its usefulness, but that installing this one without them is perfectly reasonable.
It seems that Recommends is the best option for qemu-kvm.
Note that you can use the --no-install-recommends option with installing the libvirt package.
According to the Debian policy:
Recommends
This declares a strong, but not absolute, dependency.
The Recommends field should list packages that would be found together with this one in all but unusual installations.
Suggests
This is used to declare that one package may be more useful with one or more others. Using this field tells the packaging system and the user that the listed packages are related to this one and can perhaps enhance its usefulness, but that installing this one without them is perfectly reasonable.
It seems that Recommends is the best option for qemu-kvm.
Note that you can use the --no-install- recommends option with installing the libvirt package.