eval = relating to time (obsolete) medieval = (middle ages) primeval = (dinosaurs) In this case he is saying his friend is old fashioned.

positively = absolutely ＝ 十分 primeval = we don't use this word often, but it means something like 猿人的 （不文明的）

"You are positively primeval." = you are extremely primitive(-minded). Belle is saying that Gaston has such a backward way of thinking, that he must be from the prehistoric age. No modern man would think like that.