What is believed to be the energy generator for a quasar?
A supermassive black hole.
There are supermassive black holes at the centres of many galaxies. These will have an accretion disc which is a disc of material spiralling into the black hole. If there are large amounts of material falling into the accretion disc, it will get heated by friction and the gravitational effects of the black hole to the point where it emits vast amounts of energy. This is the energy source of a quasar.