I would say that usually those top three words are used in the same way, although it probably depends on who you talk to. When I think I alternative medicine, complementary medicine, etc. I think some modality that is not related to western allopathic medicine that is used to combat disease. A good example is Chiropractic. Chiropractic (of the straight flavor) believes that if you manipulate the spine you can cure disease. That is a alternative treatment in the disease model.
Preventive Medicine, according to me, is something done to increase the potential of the human body to fight disease. In the Disease+Host=Illnesss model of Osteopathy, it would be the Host component. That would include anything from Diet, Exercise, OMT, Psychotherapy, maybe even Asprin therapy to some extent, etc. Pretty much anthing that does not attack a pathological problem but rather strengthens the host in which pathology can reside would be termed Preventive Medicine in my book.
Later. 🙂