in water fall model there is a demarkation among every 
module in the life cycle.the start of the next phase will 
occur at the completion  of the previous phase.

in the case of V-model the development and testing 
processes will occur simultaniously to save the time as 
well as money.

In Waterfall model ,the ouput of each phase will become 
input to the next phase...we can't go back in this 
waterfall modal and this work fine where there is no 
frequent change in requriment. 
Where as in V- Modal testing and developing goes parllely...
while doing verification we can aslo do the validations...
we can desing the testcases for user acceptance when the 
requirement analysis phase is going. we can design the 
system design testcase when the system engineers analysis 
the user requirements..we can design the Integration test 
design when the architecture design is carried out and we 
can design the unit test cases when the module design is 
carried and after all these completed then coding phase 
will be started...once it is done the validation phase cums 
in to picture. we will execute the test cases for which we 
have designed already...in each phase like first unit 
testing then integration testing,system testing and user 
acceptance testing.

water fall model testing is performed  under low priority 
activity.If any defect  you  can identify the risk,time and 
cost of the s/w is very high.un realistic testing and 
inadiquatetesting perform.

v-model testing is performed corresponding parallel testing 
activity.If any defect you can identify the earlier stage 
so risk,cost and time very low.The expected s/w product 
deliverd.