FKM is the designation for about 80% of fluoroelastomers as defined in ASTM D1418. Other fluorinated elastomers are perfluoro elastomers (FFKM) and tetrafluoro ethylene/propylene rubbers (FEPM). All FKMs contain vinylidene fluoride as a monomer. Originally developed by DuPont (Viton), FKMs are today also produced by Daikin Chemical (Dai-El), 3M's Dyneon (Dyneon Fluoroelastomers) and Solvay-Solexis (Tecnoflon). Fluoroelastomers are more expensive than neoprene or nitrile rubber elastomers partly because they provide additional heat and chemical resistance.

Source: http://en.wikipedia.org/wiki/FKM