Could someone put me right on servos.

I have a mill which I retrofitted some years ago which has a standard 3 phase spindle motor of 3hp. I have a 7hp motor and a VFD on the shelf which I intend to replace it with at some point in time. Lately though, with the price of servos and drivers becoming more affordable, my thoughts have been going in the direction of using a servo instead that would provide a reasonable power. I don't know enough about servo power outputs to take this any further and would like some help.

On the face of it a 3KW servo is the same as a 3KW AC induction motor, but I know it cannot be as simple as this, or is it? Ignoring gear reductions and servo drivers for the moment, how does the power output rating between the two units compare. Thanks.