I have a Chinese visa but I don't have a visa from Hong Kong. Can I go to China from Hong Kong airport without the visa? I am getting convenient flights to Hong Kong.
The quick answer is "Yes, but you'll need to do different things depending where you're from".
Hong Kong has different entry/visa requirements from China, in general it's much easier to get into Hong Kong. You can see the different country requirements on this page. So the chances are you will be able to get into Hong Kong. Then you can get the train, ferry or even a car into China.
If you don't meet those requirements, or if you don't want to enter Hong Kong, you have another option. You can get the ferry to China directly from the airport without having to go through HK immigration. Just follow the signs for 'Ferry to Mainland'. They'll even get your bags for you and put them on the ferry. Here are the official details.