"Excess" is a noun, while "excessive" is an adjective. The "-ive" ending is a standard Latin/French suffix for forming adjectives.

"Excessive" means immoderate; more than normal. Example: He drinks an excessive amount of alcohol; he is an alcoholic. Example: He is spending an excessive amount of money. He is very extravagant. "Excess" means "more than a prescribed amount". Example: Each passenger is only allowed 20 kg of baggage on board the aircraft. Anything above 20 kg is considered excess baggage. Example: John and Mary do not want any children. They think that children are excess baggage in life. Example: Any excess calories beyond 1800 a day will make you put on weight.