Yes, kevlar offers only limited protection against knives.
You can think of kevlar as many layers of nets. The nets "catch" bullets, and as the bullet hits each layer of netting it loses energy. After many layers the bullet is out of energy and ceases to be a threat. Kevlar is able to do this because the fibers are very strong, and can absorb the energy of the bullet without breaking.
This only works because the bullet only has the initial energy of the gunpowder. Once the bullet is fired from a firearm, it ceases to gain energy and only loses energy. A knife, on the other hand, has CONSTANT energy behind it (someone is pushing/slashing with their hand). So kevlar cannot "neutralize" the energy of a knife, since there is constant energy being pumped into it.
Another issue is that knives tend to be pointy, and these points can find their way in between the layers of nets that make up kevlar vests. This is unlike a relatively blunt bullet, which gets caught in the nets. In this way, kevlar offers more protection against slashing attacks than stabbing attacks.
Kevlar (and similar materials) can be combined with aramid fibers to offer both bullet and knife protection. Still though, just as how bulletproof vests are not 100% bulletproof, stabproof vests are not 100% stabproof and knives can still penetrate them given a lucky angle or enough force.