I'm Bangladeshi citizen & currently study in KL. Can I apply for Hong kong visa for visit purpose ?

Dear, yes, you can apply for it in KL. And you should submit those documents containing your passport and its photocopy, the application form and a recently-taken color photo, proof of financial support, travel plan to HK, and the flight itinerary. Afterwards, submit the application to the embassy of PRC in Malaysia or HK Immigration Department and wait for the result.

Does it mean like hotel booking & air ticket also? how many days it will take for the processing ?

Yes, you need to submit hotel bookings and air tickets. If the embassy in KL can deal with your application on its own, you only need to wait four working days. If they transfer your application to HK side, you may need to wait for four to six weeks before the result comes out.