For heating water in a kettle both the electric kettle and stove stop are pretty energy efficient.
The electric stove top will usually use jut a bit more electricity than the kettle but with the electric stove top it will heat the kettle and water up in the kettle much faster so in away it either evens out or uses less electricity and you get the water heated up faster.
With a gas stove the energy will actually cost less per BTU to heat the water up so I would say the stove is much more energy efficient at heating water up in a stove top kettle than an electric kettle would be.
The stove top kettle takes less time to heat the same amount of water up so it would be the most energy efficient form of heating the water.