I have installed Dunlop GRANDTREK WT M3 P275/45R20 XL - 110V Winter Tires on my Q7 but the vehicle slips terribly even in little snow. What should be the tire PSI for one passenger and with full load? I have set it to 36.

Also the Tires makes a lot of noise. (

 

The Q7 is a fairly heavy vehicle with adequate(sometimes more so) power a very capable braking system, so it's not difficult to induce wheel slip on a slippery road surface. Any chance there might have been some ice underneath the snow? Even the best studless Winter tire is going to have some slip on icy surfaces.

Granted, I don't have any direct experience with the exact tire that you mentioned. But I can see that you've maintained the factory tire size with your Dunlop Winter tires instead of downsizing to a narrower tire/wheel. There's nothing wrong with doing this but there are some trade-offs. Maintaining the wider tires should be beneficial when driving on dry/wet roads as well as having better cornering stability. They should also do well on packed snow surfaces. But the wider tires will have more slip inducing resistance if you find yourself attempting to 'cut a path' through deeper snow(~6"+).

As for your tire noise, again, I don't know much about your Dunlops other than the fact that they appear to be aggressive studless Winter tires. It's not unusual for Winter tires to generate a little more noise than all-season tires but they obviously shouldn't be loud enough to drowned out your sound system.