The issues seen in the traditional waterfall model gave birth to the V-Model, it was developed with an intention to address some of the problems found in waterfall model.
As you can see that in waterfall model defects were found very late in the development life cycle because testing was not involved until the end of the project.
In V-Model testing begins as early as possible in the project life cycle, it is always a good practice to involve testers at earlier phases of product life cycle. There are variety of test activities that need to be carried out before end of the coding phase. These activities should be carried out in parallel to the development activities so that testers can produce a set of test deliverables.
The V- Model illustrates that testing activities (Verification and Validation) can be integrated into each phase of the product life cycle. Validation part of testing is integrated in the earlier phases of the life cycle which includes reviewing end user requirements, design documents etc.
There are variants of V-Model however we will take a common type of V-model example. The V-model generally has four test levels.
1. Unit Testing
2. Integration Testing
3. System Testing
4. Acceptance Testing
In practice the V-Model may have more granular test levels like unit integration testing after unit testing.