Why doesn't the extra electron in n type semiconductor due to doping by a pentavalent impurity create a hole?

To answer your first question, the pentavalent dopant atom would have a net positive charge - however this is mostly screened by the rest of the lattice. As a result the electron is very weakly attracted to it, and it is very easy to excite this electron to the conduction band. The energy level of the dopant is much closer to that of the conduction band than the valence band.

This is also why it doesn't create a hole in the valence band; because that electron did not come from the valence band. It stays in the dopant energy level within the band gap.