I place a buy BO order for lets say 300 quantities of a stock, with target 1 and SL 0.5. The order gets partially filled for 200 quantities. Now, volatility comes and stock jacks up by rs 1. So my target for 200 stocks is reached. What happens now? Will be 200 quantities be closed with the the rest of the order of 100 quantities cancelled? That means will the entire BO be closed by cancelling the pending order and exiting the target achieved order.

In your example, when your target for 200 stocks is reached, the corresponding SL gets cancelled. But your original order which has 100 pending will remain in the system until executed or cancelled.

So we can cancel the pending partial order, am i rite?

Yes, the main order.