What is the difference between granadilla and passion fruit?

Maybe, depending on where you are. Granadillas are fruits of the plant Passiflora ligularis. In most places, "passion fruit" is used to refer to fruits of the (closely related) plant Passiflora edulis. However, in Australia and most of Africa, Passiflora ligularis fruits are also called "passionfruit".

There are several hundred species of Passiflora (Latin for "Passion flower'), and many of them have edible fruit, so what's locally and logically referred to as "passion fruit" might come from any number of different species.