A full bore (or full port) valve is one where the hole in the ball is equal in diameter to the hole in the pipe. In other words, if you were to look down a piece of pipe which also contained the valve, you would not notice any constriction at the location of the valve.
A reduced bore ( also called a standard bore or port) valve is one where the hole through the ball is smaller than the hole in the pipe. In some valves such as the Jamesbury 5000 series, there is a gradual narrowing so that the valve almost looks like a vena contracta. In other valves, such as the reduced bore Jamesbury 4000 series, the reduction is simply a shoulder.
Often, the reduction in diameter is to the next standard size. For example, a 2" (nominal size) reduced bore valve would have a 1.5" bore in the ball valve. A 1.5" (nominal size) reduced bore valve would have a 1.25" bore in the ball and so on. This comes from a rule of thumb which actually coined the term "standard bore" as much as the desire for modular design to allow the same ball to be used in one size of full bore valve and another of a standard bore. Long time ago, the engineers noticed that to get good control, very often the solution was to use a full bore valve of one size smaller than the pipe you were using. Now, this arrangement required reducers on either end of the valve. Someone then came up with the idea of integrating the reducers in to the valve and the standard bore valve was born.
A V-port ball valve has a V-opening in a shell-formed ball section. Controlling the flow from fully closed to reveling more and more from narrow end tu full V to regulate a flow.

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