EA's Version Control functionality does not support this. As you've noted, what actually gets stored in the version control system are XMI files, and XMI is a proprietary-extension format, which means third-party tools won't be able to make much sense of the changes between two versions.
What you can do is use EA's Baselines, which do include the ability to do visual comparisons between the current model and a baseline (but not between two baselines). This includes added/changed/deleted model elements, as well as purely visual changes (moved elements) in diagrams.
You can use both Version Control and Baselines in the same EA project if you wish. My recommendation is always not to bother with Version Control at all and only use Baselines, but the best solution does depend on your specific needs.