What kind of visa is required to enter Hong Kong, then Hainan, then Hong Kong again?

We will be flying from the US to Hong Kong, stay for 3 days, then fly to Hainan island for a week. After that we'll return to Hong Kong for 1 night then go to Hanoi and beyond.

Do the 2 entries into Hong Kong require a multi-entry visa? (Of course we'll have our passports.) Is a Chinese visit required to visit Hainan island?