I have Pakistan passport and holding valid tourist visa to Canada. Do I need HK transit one?

My first response to this is no, but you cannot leave the airport at Hong Kong. Normally a transit passenger does need a visa to pass though an airport--all that is normally needed is proof of a ticket departing that airport. This is normally checked by the airline before they allow boarding.
It would be wise to do a quick check regarding transit visas at any airport if you uncertain, as reqirements do depend on your nationality. As an Australian, I have never had a problem here, and I doubt you will. Having a Visa is not proof of an ongoing ticket, so your real problem will be getting to the Phillpines if you do not have a flight booked out of the Phillippines! They won't let you board at Hong Kong without that ongoing ticket
Steve G