Online Chat - Approach to keep History

I am working on Online Chat first time in Hire and Work Website where Employer will post project, it's a kind of getting work done from Freelancer. I am told to keep Chat history also. So far I though to keep it using below two approaches.

1. Save Messages in Database
2. Save Messages in Xml file

Below were the reasons for keeping things in XML.

1. There will be always 1-1 chat.
2. Get rid of millions of messages in single Database Table. It may cause performance issue to load filtered data from millions of records.
3. Each time an Employer will do discussion for their new Project. So previous chat will have nothing to do with old project.

Am I doing it correctly? or there could be another approach also?

via Chebli Mohamed