Cavitation can be caused by a number a things.

Damaged Propeller. Air being introduced into the propellers path (eg: transducer, boat design) Propeller design. Basically, it is unwanted air which has been trapped in and around the propeller blades which could possibly lead to a complete loss of propeller grip. Cupping a propeller can help reduce cavitation.

Ventilation

Ventilation on the other hand can be uncontrolled ie: Engine Height set too high (sucking air from the waters surface) or controlled air introduced around the propeller blades which can increase motor response by allowing the motor to gain extra revs at lower speeds. Holes are drilled into the propeller hub to allow exhaust gases to flow out. There are other applications where a protruding edge around the outside of an outboard gear-case has been designed to cause turbulence on to the propeller blades. In a racing environment the exhaust of the engine may flow completely over the propeller blades.