Can I drive a commercial vehicle with a normal drivers license?

YES, You require a commercial driving licence. Conditions to apply for a commercial license are below.
1. First, you should obtain a license for private car (non-transport).
2. You should wait atleast a year to apply for commercial license.
3. Minimum qualification for commercial license is 8th standard and you should be able to read and write.
4. First you have to apply for LL (Just a signature. No signal test) which will have 6 months validity.
5. After 30 days (minimum) you can go for the driving test.

Can I drive a commercial vehicle (a taxi),i.e. a car with a yellow number plate with a normal drivers license if I do not carry a passenger with me? Elaborating the case: I'm planning to own yellow board sedan and employ a driver with a commercial license to act as the cab driver. My query is whether I can drive the same once in a while to get from one place to another provided I don't carry another passenger with me. Is that possible in any way??

NO, you cannot, you have to obtain a Transport license with badge if in Karnataka