Hi, I have a Chinese student visa on Pakistani passport and want to visit HK.

First, provide copy of passport, the application form affixed with a passport-style photo, proof of your financial support, transportation and accommodation arrangement, and your travel plan.
Next, you can post the application to HK immigration department.
Address: Receipt and Despatch Sub-unit
HK Immigration Department
2/F, Immigration Tower
7 Gloucester Road
Wan Chai, HK
Usually, it needs 4 to 6 weeks for the application process.