What is the difference between cavitation and flashing?

Flashing is the first stage of cavitation. Flashing occurs in liquid flows when some of the liquid changes permanently into vapour. This is brought on by a reduction in pressure forcing the liquid to change to gaseous state. Bubbles can suddenly collapse or implode as the pressure increases down line, producing Cavitation.

Cavitation:

This phenomenon will occur when the vapor pressure goes below the outlet pressure (P2) and recovers above the outlet pressure after some time. 

Flashing:

This phenomenon will occur only when the vapor pressure goes below outlet pressure and will not recover above the outlet pressure.