When you have several version of a (kernel) knowledge base, you are able to compare knowledge base versions by opening both knowledge bases and choosing Tools>Compare Kernel KB’s. (Please note this is only enabled when you have opened two kernel knowledge bases...).
After you have selected this option, for each knowledge base you will only see the frames in that knowledge base containing differences compared to the other knowledge base. Note that this can be any difference, so also only the @ENGINEER or @LASTCHANGED attributes. The Web Browser should be automatically opened to report the specific differences (if not open the Web Browser with View>Web Browser). The information identified as Source is always the knowledge base that is not being in focus at the moment.