A knife gate valve is just what it implies. Knife gate valves were originally designed for the pulp and paper industry. Stringy pulp would impinge between the wedge and seat of a normal gate valve and prevent flow shut-off. The knife gate valve was designed with a sharp edge to cut through the pulp and seal. Due to this, the knife gate has found a home in slurry, ash, and other systems where impingement is a problem.
'Standard' gate valves use a solid, or flexible wedge design to seal. the wedge is forced into a V-shaped seat and sealing is accomplished by virtue of the brute force utilized to drive the wedge into the seats. Thermal binding is an issue with wedge gates.
Parallel slide gate valves utilize 2 parallel seats as opposed to the V-shape of wedge gates. This design bases sealing not on torque, but on the spreading action of the 2 disc faces as the valve seats bottom out. It also utilizes the motive pressure of the media to seal the downstrean disc face. In theory, the higher the pressure, the better the seal. With slide gates, only one of the two seats are actually sealed. This type valve is being used more often now for high temperature steam systems as thermal binding is not a problem.
In terms of advantages/disadvantages, knife gate valves are generally limited to ANSI class 150. They are cheap, easy to actuate, and light. sealing is by either metal to metal, or an elastomer o-ring seal. They are unidirectional shut off valves. Wedge gate valves are available to ANSI class 4500 and provive reasonable shut-off in both directions.

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