Many universities conducts the viva for different levels of research works. However, whether they follow same caliber or different caliber for different levels during the evaluation becomes a puzzle.
I believe there are different levels of importance, and criteria for each level. For the Bachelor level, it may just be that it is necessary to get an idea of how to execute an idea, collect results, analyze, and discuss. It may be important for them to know how to write it. However, at the Masters level it becomes more important for them to be able to write competitively. To generate good ideas, execute, and be even able to publish their works and present them to other scholars. Now, at the PhD level, the expectations totally differ. They are expected to create new knowledge of sorts. The expectations are higher. Their ideas, or execution must not only be original, but relevant. It therefore implies that the level of evaluating a PhD thesis cannot be the same as a Masters, or a Masters to a bachelor one. In fact, the PhD student must not only publish but publish in highly reputed journals. The quality of work and ideas produced increases as the educational level increases as well. I hope this answers your question to some extent.
Nagendrakumar Nagalingam I believe each university has it's own framework. The pattern i described is in general. However, various universities go about these assessments differently. You can also ask at your own university and then check other universities and compare assessment matrix. You may find similarities and differences accross different levels.