Energy efficiency of microwave and electric kettle for heating water
According to the folks over at treehugger.com, they did a study to measure which of the stove, the microwave or the kettle was the more efficient in terms of use of electricity to heat a standard mug of water. This doesn't directly answer your question as you are unlikely to boil 350 ml (standard cup is 250 ml) of water in your kettle or on the stove, but it gets the point across.

A summary of their findings is that it took the electric stove 0.11 kWh, the microwave (900 W) 0.07 kWh, and the kettle 0.04 kWh. This indicates that in terms of energy usage the stove is 30.5% efficient, the microwave is 47% and the kettle is 81%. 

The clear winner is the kettle!