businessman.I have a Chinese visa for canton fair 2016 April can I also go for HK with it?

No, you cannot go to HK with it. You should apply for a new entry permit to HK ahead of your trip.

How do I apply for a new permit to go to HK

You should submit the relevant documents to the PRC embassy and then wait for the result. The documents contain your passport, a copy of it, a filled application form, a recent bare-headed photo, transportation arrangement proof, hotel reservations, a financial support proof, and a travel plan.

How long does it take there?

If your application is processed by the embassy itself, it usually takes 4 working days; but if is transferred to HK immigration department, 4 – 6 weeks are needed.