What is the major difference between Mexicans and Central Americans?

Many tend to say central Americans are more Indians, darker , and uneducated . for example i heard Mexicans dont like Salvadorians , why is that? in my personal opinion Salvadorians are more americanized than any other Latin culture . also in el Salvador there is only 1 public university and the rest are all private , and i been to that country and what i seen is that mayority of them graduate .