Pervillei is different because it is not cross fertile with other Nepenthes species. It also has morphological differences compared with other Nepenthes species, particularly in relation to its flowers and seed.
Cross genus fertility does not exist, as far as I'm aware.
What constitutes a species is an interesting discussion. There is a parrot that is found all along the east coast of Australia (which is, lengthwise, a phenomenal distance). The colouring changes gradually as you go from north to south. However, the northern parrots can interbreed with the ones from the central region, the ones from the central region can interbreed with both northern and southern forms, but the northern and southern forms cannot breed with each other.