Zener Breakdown

Electric field in the depletion layer increases to the point of breaking covalent bonds and generating electron-hole pairs

Large current can be obtained without increase in reverse biasing voltage

Avalanche Breakdown

Reverse bias increases the electric field at the junction.

Increased electric field increases force with which carriers collide with lattice atoms.

Impact ionisation - At the 'critical energy' point, the collision between the carriers and atoms ionise the atom (removes a carrier, such that it is free)