In ball valve is Bernoulli principle used to estimate the values  flow rate & pressure drop estimation
And what will be the difference with ventury flow path & Straight flow path
Straight flow path (Full bore) has the same pressure loss as an equivalent length of straight pipe.  Maybe less, as the ball has a better internal surface finish than pipe. 
It is more customary for a ball valve to have a "standard" port, which is typically one size reduced.  There is a theoretically larger loss.  In practice, the loss is about the same as one experiences fron an elbow. 
Full bore is really only required if something solid, such as a pig or an instrument probe, has to go through the ball.  It's meaningful as long as there is not a control valve in the system running slightly more closed, absorbing the same energy that was liberated by opening the ball valve port a bit. 
Meanwhile: the full-port valve costs a lot more, and the actuator torque goes up by some greater-than-1 exponent.
My point is: Most of the time a full-port valve is unjustified.  Probably 95% or more. Digester Blow, Peelers, Pigged pipelines, Instrument tap valves, to name a few apps.  

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