They're actually quite different.
Definite = certain, for sure; also well-defined
Definitive = final, settled, cannot be argued with; the best of its class
Tom and Sue are getting married? Is it definite? (is it for sure, is it certain)
The definitive court case ruled that two men can now marry each, as can two women. (cannot be argued with)
It is not definite if I can take vacation next month. (not certain)
The definitive (the best and it cannot be argued) place to go is definitely (certainly) the Isle of Capri off the coast of Italy.