Dragon xp is multiplied by the ratio of hero level to pet level. I did some experimenting and posted my results
here.
I did not do any testing on how summoned units factor into it. If you are technically inclined at all, it's fairly easy, if tedious, to do some testing for yourself. You could also double check my work.
Use winrar or similar to open data.kfs (found in the data folder) and edit pet.atom with any text editor. There you can change the experience values for the skills to something like 1000. Run a bunch of tests and with a number that large, it's easy to see if and how it changes. A backup of the original is probably a good idea.