Adriaan,
LV is for rifles 10.5 lbs and under, HV is for rifles 13.5lbs and under. You can use an LV in HV if you like, which is very common. Pretty much what most of the shooters do that have only LV's, like me for example.
One other thing, you can shoot an LV in Unlimited and in Sporter (essentially LV rifles, 6mm and fatter). This covers all the group shooting classes with one rifle.
Rick