I am Pakistani citizen and have valid Canadian visa till August 2014. I want to go back from Canada to Pakistan and want to spend three weeks in London to visit my daughter and have confirmed ticket to go back to Pakistan. Can I get exit visa?
No. Neither Canada nor Pakistan issue exit visas. Therefore, you do not need one and cannot get one.
According to Wikipedia, exit visas exist in: Saudi Arabia, Qatar, Nepal, Uzbekistan, Cuba, North Korea. You only need a visa to enter Canada.