I had started to post a long, detailed post explaining what to look for in a quality motor, but then I realized I was wasting my time. Most on RR and other websites see a long post and skip it.
So here is the semi-short answer to the OP's question about which of the three selected motors should one get. First I have used all three plus many others including Scorpion, Align, Xera (predecessor to Xnova), DJI and no name brands like NTM from hobby king. In a nutshell most work just fine if you select the correct one for your use.
The quality and performance of an electric, brushless, outrunner motor can generally be determined by its measured efficiency. Sometimes manufacturers provide accurate data on efficiency, while many don't. The efficiency is important because it gives insight into how well the motor is constructed. A neatly wound stator using minimal air gap between the stator windings and rotor's permanent magnets, having a stator core made from thin ferrous plates to reduce eddy currents, and having quality bearings which precisely fit into their seats will generally make for an efficient motor. Efficiency means increased power output for a given input. The wasted input energy or inefficiency gets turned into heat, which, along with the actual material specifications and cooling airflow, is what limits continuous and peak output over the specified duration.
The material specifications need to be examined to understand at what temperature the motor will fail or substantially lose power. The bearings should be of high quality like ABEC 3 or better. The magnet wire should have a high temperature insulation coating. A magnet wire rating for continuous operation at 200Â°C is good, 300Â°C is even better. The magnets themselves should be of grade N42 neodymium or better like N52 (not often used due to cost), preferable with an H, SH, UH or EH temperature designation. For example, a N52UH magnet is the strongest rare earth magnet available and has ultrahigh temperature properties. This type of magnet not only produces the most powerful magnetic field, it doesn't lose its field strength as much as motor temperatures rise compared to a lower strength neodymium magnet without temperature protection added ingredients.
All three motors under consideration are good choices. They all have cooling fans incorporated into the rotor housing and are powerful. Your choice comes down to how much you are willing to spend for a little extra performance and longevity. The KDE and Xnova (at least in the XTS Xnova series) are like custom made motors. They use hand wound single-strand magnet wire with 300Â° C temp rating. The tolerances between stator and rotor are tight. The top bearings are doubled up to provide long bearing life. The magnets are high temp. rated to prevent lose of power at higher operating temps, or worse, total and permanent demagnetization at the non-high temp. magnet's Currie point temperature (around 310Â°C).
The Quantum has a machine wound motor stator using multi-strand magnet wire with a 200Â°C rated coating. They use a single top bearing. The Quantums use good magnets too. However, the machine winding necessitates a larger air gap between stator and magnets. The stranded wire reduces the cross sectional area of the winding. Together, these factors will decrease the Quantum's efficiency and max power compared to the KDE and Xnova motors of the same size.
The thing is, is it worth around double the money to get a slightly better motor? I use a Quantum in my Goblin 700. No complaints. The motor does get hot, but that's a function of putting a lot of input power through it rather than being an inefficient motor.
Of the two top dogs, KDE and Xnova, I believe the KDE motors are slightly heavier throughout their lineup. That can be a good or bad thing. More mass of a given material (aluminum housing/rotor or iron core) has the ability to absorb heat during peak power usage, thereby reducing peak temperature. Of course this comes at the expense of more weight.
I personally prefer the lower weight of the Xnova. It has a great balance of power and weight. Keep in mind that RC heli motors will only experience peak power uses for a brief time period, unless you do speed runs.
One last thing to consider. There may be just as much if not more performance to be had by selecting a better/more efficient ESC and higher C rated battery than by motor selection. Good luck with your choice.