To enable you to get more information about what ClamXav is doing when it's scanning files or updating definitions, you can control its logging settings in this preference pane.
One thing to note is that logging is always enabled for ClamXav Sentry, as well as scheduled scans and scheduled definition updates.
Note for power users
ClamXav Sentry will log all files being scanned but by default, manual scans within the main ClamXav application will only log infected files. If you wish for clean files also to be logged, you can invoke the following expert preference:
defaults write uk.co.canimaansoftware.clamxav LogCleanFiles -bool YES; killall cfprefsd