Difference between carbon steel and stainless steel knives?

It depends on the type of stainless steel you are referring to. Austenite and ferritic stainless is not heat treatable in which case carbon steel could be made far harder. However..martensitic stainless steels are heat treatable in which case they could be made harder depending on the alloy contents. Generally the more carbon a steel contains, the harder it can be made. Chromium; a key ingrediant in stainless steels, can also increase hardenability.