Hong Kong mutliple entry?

We intend to tour from Beijing, down to Xian, Hong Kong, then Shanghai. Does entering and then leaving Hong Kong count as a multiple entry, or is this trip a single-entry visa?

Also, I would like to apply tom'w-- we have the airline tickets (including return flight) and the first hotel in Beijing, but I still have to book hotels in Xian and the other cities. Is there any way to get it before I do that?