what is the diffrent between on demand scan and full scan on mcafee ?

On demand scans are configured for the most likely file types and locations that would include viruses/malware.  Full scans include all file types and locations.  As a result full scans take much longer to run and most users rely on on demand scans (or scheduled scans) which are faster and so generally more efficient in terms of detections over time taken.

Your report seems fairly normal as if the on demand scan picks up anything it will be deleted/quarantined so any full scan following this would show no infection as they've already been dealt with.  Check the dates that your last full scan ran, if it was after the on demand scan with the 8 detections then this is fine, if it was before then 8 infections had been picked up since the last full scan and you might want to look at how that happened.

thanks for your reply, but it is strange that on-demand scan detect 8 but it did not show what did for that, it did not delete or clean that virus detect even not mentioned in log .