Here are a few suggestions:
If the run created the DRC Summary Report, compare the "RULECHECK RESULTS STATISTICS" section for each run. Perhaps you can create a script to compare this section from the two runs and output the information you want.
Save a CSV file from RVE. You can do this with File > Save Results As, or from the command line (see the manual). The CSV file will have line a for each result, but perhaps you can use this to get what you want. It might be easier than comparing the actual results databases.
Create an HTML report with output for just one result in each rule. In DRC RVE, select View > Tree Options > Group By > Check to sort the group the results by Check only. Then select Tools > Create HTML Report. In the dialog box, set "Number of results to report" to 1, then click OK to create the report. The report will list only one result for each check.
You may also want to turn off DRC Keep Empty, it is set to YES by default.
I hope one of these will help you,