I am travelling from Pakistan to the USA on a tourist visa with my uncle, aunt and their son (my cousin). Is it allowed to travel without my parents from Pakistan to the USA, given that I have a Pakistani passport (Pakistani citizen) and a US visa? I am travelling with Qatar airways. If the airline allows one to travel without parents, will immigration not allow me to travel without parents or will any additional documents be needed?
Yes, it is allowed for minors to travel without their parents.
However, various officials along the way may want to verify that you have your parents' permission to travel. It is not certain that you will be challenged, particularly if your entire party looks like a family going on vacation, but since international child abductions are serious business, you should be prepared for it all the same.
You should carry a letter in English, signed by both your parents and stating that they are leaving you in the care of your uncle and aunt, named such-and-such, for a trip to the United States on such-and-such dates.
Contact your local border authorities to hear whether they recommend/require any formalities such as getting the letter notarized.