Ventilation is basicaly air or exhaust gasses that go thru the propeller blades where the air or exhaust has positive pressure.
Cavitation is gas with a negative pressure (vacuüm).
Ventilation could be hurt your performance, or you could benefit form it, mostly controlled.
Cavitation is mostly destroing your propellerblades.
If your propeller is not sealing well with the gearcase (in case of exhaust thru propeller) you get some ventilation. This will reduce the grip of the propeller in the water, with the correct propeller is this minimalized.
If you go driving backwards, all of the exhaust gasses are escaping out of the propeller and right thru the propeller. This will give you very bad performance in reverse gear. Because of this, manufacturers have made good solutions for this problem, this is important for you if you need good performance in reverse.
Sometimes you need a good torque to have your boat planed, however if you open the throttle fully, the engine will not come above a certain RPM. To increase this RPM, you need to control some air or exhaust gasses thru the propeller blades. Some propellers have a smaller hub or holes in front of each propeller blade, Mercury even has PVS, Performance Vent System, that has ventilation holes that are replaceble with different opeings, or even closed.