In that case, do i need to apply for separate visa? if so, where should I go,and what documents?

Raza, you can visit Chinese embassy in Korea to apply for a visitor type for your HK conference. It requires:
1. copies of your passport information page
2. finished application form (ID1003A）
3. a recent passport photo
4. flight itineraries and accommodation in HK
5. financial proof, recent six months bank statement, pay slip or employment certificate.

You'd better apply at least one month ahead of your trip. If the embassy issues it, it takes four working days. However, if your application was sent to HK, then you should wait four to six weeks.