What is this tube-like thing protruding from under an A330's wing?
It is the port for mid-air fuel jettison.



(Image credit: http://opilotoprofissional.blogspot.hk/2011/08/consideracoes-sobre-overweight-landing.html)

The A330 is a long range airliner, capable of serving flight routes that are 10~12 hours long. It is equipped to dump fuel while in-flight to reduce its landing weight.

The A320 is not a long range airliner, thus there is no need to install such equipment on it.