Q:

Class AB amplifier

What role does Rv play in this AB class amplifier?

A:

This is a class B amplifier: -

Your circuit is a class AB amplifier: -

Rv adjusts the bias point of the two transistors so that T1 and T2 are always conducting a little bit of current - this avoids excessive cross over distortion: -

See also this article, Crossover Distortion in Amplifiers, for more information.
Rv modifies the volt drop across the two series diodes. Remember that diodes are not just fixed 0.7 v devices. The forward volt drop can be adjusted so that the base-emitter junctions of each output transistor are conducting 1 mA or so, placing the transistors in a much more linear region of their characteristic at the expense of a sending DC current thru the transistors (an increase in power dissipation).

