The Yanmar diesel has been around for along time and John Deere has had a long relationship with them. That's the only company that Deere uses in their Compact Utilities, Mowers, and gators. It is really a bullet proof engine. That's in the XUV 855D.
The XUV 625i runs a 24hp Kawasaki engine. Again, another bullet proof engine. Very dependable and a bit more forgiving when maintenance isnt done as regularly as it should.
The XUV 825i has a Chery Engine, which is a Chinese made engine. Chery has been used a LOT out west in Sand Buggies and the like. The reason Deere chose this engine, is a couple of reasons. 1.) Its been proven out west that it can tolerate some harsh conditions and 2.) It's a low maintenance engine. Now with that being said, there are only so many things maintenance wise that a dealer can do with those engines with out having to replace it. So far, In our dealership, we have had really good luck with those selling about 65 825i's a year.
And, the XUV 550 has a Briggs & Stratton Vanguard V-twin, air cooled, carbureted and is 570cc, and produces about 16hp.
I've got a Yanmar 3TNE74 in one tractor (X595se) and a 3TNV76 in another (2305). Both are basically larger displacement versions of what's used in the 855D and they are indeed very good engines.
I certainly would not let the Chinese Cheri engine in the 825 scare me. It's used in some of their automobile models so it's a high volume engine that's been thoroughly engineered and is a proven performer.