Welcome to IFI Warhammer, this is a good place to learn about smithing. The majority of smith's use a type of peen hammer, straight, cross, diagonal, ball as their regular hammer. I use a cross peen, the "back" of the hammer head is a narrow wedge of steel that is horizontal to the handle of the hammer. Many smith's use this type. It does not have to be a particular style or type of hammer to make knives with.
Here is a picture of my primary forging hammer.