Guidance on visa for Hongkong

Hi,
Do i need it for Hongkong as I have a dual citizenship US/Pakistani? But I am a student in Xi'an and my student residence permit is on my Pakistani passport. Can i exit china using my US passport as US citizens to enjoy the free entry for Hongkong and return on my Pakistani passport? Or do i need one?
Can i get one for Hongkong upon arrival?