You can first find the revisions of the tags:
svn info /tags/19_08_2011_PR/ | grep 'Last Changed Rev'
Last Changed Rev: 14082
svn info /tags/31_08_2011_PR/ | grep 'Last Changed Rev'
Last Changed Rev: 14382
You're not really looking for a diff, you are looking for a log. So do a verbose log between those revisions and you will get the commits including changed files.
svn log -v -q -r 14082:14382 /trunk