I have a 2006 Polaris Hawkeye 300cc that has had a huge loss of power.  I've cleaned the carb and checked the jets, changed the fuel filter, air filter, spark plug and have done an oil change on it.  Can any one help.  

have you checked the compression?

also if you have the pvt transmission, ( i think it does have it)
the belt could be worn.

have you had the steering stem replaced on yours yet?
there is a recall on all polaris hawkeye atv's because the steering stem (shaft) breaks.
many people injured because it came apart.