All US citizens have to pay taxes on the worldwide income, so basically, yes, you have to pay US taxes.
However, after you meet some tests, you can exclude from your income unto $95,000 USD and a very large amount for your housing expenses--after you meet the proper tests. This has the effect of many people not owing the US government (or their state government) any income taxes. And yes, many states will still consider you resident for tax purposes if you maintain US bank accounts, driver's license, voting, property, etc.
Unfortunately, if you are only coming for 9 months, it will rather difficult to meet this test. You'll likely be paying HK taxes and US taxes, though you'll get a small deduction for foreign taxes pay.
In addition, you need to report if you have any bank account that AT ANY TIME during the year has > $10,000. If you have two accounts that are $5,001 then you also have to report (it's $10,000 in the aggregate at any time during the year). This is called an FBAR.
IRS Publication 54 is designed for Americans living overseas. Probably worth a look through.