How can I decrease power losses of H-bridge with 4 N-channel MOSFETs?
You are going to get yourself in trouble - but don't let me stop you.
As has been commented on, you need what are called high side drivers. In theory, they look something like



simulate this circuit &ndash; Schematic created using CircuitLab
V2 is usually based on a charge pump, which will provide an isolated voltage with relatively low current. Of course, if you're doing high-frequency PWM, the current may not be all that low, but that's up to you. You have not passed along any actual numbers, so there is no way to advise you.
At any rate, when the MOSFET is turned on, and Rds is low, so will Vds. This means that the gate voltage will need to be about 12 volts above the drain voltage V1. (12 volts isn't a magic number, but it's a good starting point for producing a gate voltage which will drive a standard MOSFET into saturation.)
You'll need separate voltage sources for Q1 and Q3.
You've imported a circuit which works at levels more than 100x smaller than your actual requirements, so we've no reason to trust your &quot;20V&quot; value for V1.
You obviously have no experience dealing with switching at these currents, so let me warn you that fast turn-on and turn-off (especially turn-off) will have major effects. Whether you make a pc board or some kind of discrete breadboard, layout details will be critical, and you MUST keep leads as short as possible. And your idea of &quot;possible&quot; may very well be insufficient. Just a warning.
As for flyback diodes, I suggest you go to a site such as digikey.com and do some searching.
Depending on what you use as a gate driver, you may well need to pay close attention to heatsinking your MOSFETs. A classic tradeoff in this sort of circuit is switching speed (turn-on/turn-off times, as well as frequency) vs power dissipation. Slower gate transitions will decrease switching noise and transient levels at the expense of increased dissipation in the FETs.
I expect your experimentation phase will be painful. You will almost certainly kill a number of MOSFETs, and if you're unlucky you'll damage your power supply and maybe start a fire. 200 amps is a level where all sorts of things can go wrong. Frankly, I think you'd be better off buying a commercial unit.
But don't let be stop you.