Cargo holds are pressurised. There are also parts of the hold which are heated (for example to carry pets, and other items which cannot stand cold).
They need to be pressurised because the passenger floor of the aircraft is not a pressure bulkhead, and cannot by itself withstand the pressure differential (that is, it would collapse if exposed to the outside air pressure).
I am sorry, I don;t know if the cargo hold is maintained at a different pressure than passenger cabin. The floor doesn't have to be a pressure bulkhead to maintain small differences in pressure. Properly designed smoking rooms, such as those in NRT, have a lower pressure inside than outside (i.e. in parts in the building, but outside the smoking room) to prevent smoke from escaping the smoking room.
While it does get cold in the hold, many people, myself included, have carried items such as wine, perfumes, cosmetics etc etc in the hold and they are just fine at the other end
Aircraft hold are specifically given some heat for this reason, passengers carry liquids in their luggage which they don't expect to be frozen!