kevlar is a material that is flexable, yet extremely strong. It stops bullets do to its extraordinary tensile strength. it does, however have a low shear strength, which allows for cutting.
the reason it works so well for bullet-proofing is the weave allows for ajoining fibers to add their strength and to not allow the assualted strand to move out of the way. it is a very tight weave, if it were not, the fibers would just part and allow the bullet to pass through.
knife proof vests have different materials that are hard and stiff that foil the relatively powerless thrust of a knife attack and resiste cutting forces.
This other material can be a resin compound so that it is mixed with the kevlar cloth (think fiberglass).
It can also be another type of woven material, like a screen or modern version of chain-mail. the latter types of constructions allow for flexability, whereas the resin type of system would be very stiff.