Everything is more or less well explained in previous posts, and those who didn't know it by now may have learned the difference between personal and private.
However, it should be noted that there is someone else who can technically read your personal messages, and this is the BPIP extension. If you use it, pay attention to the following :
We don't log your IP address when the extension contacts our server but if you don't trust us you should use Tor or VPN. The extension can technically read any data from your BitcoinTalk session, including your PMs, but it doesn't do so. It collects user IDs from the page you're looking at - e.g. a list of posts in a thread - and sends that list of IDs to bpip.org to get info about those users, and only does so if you turn the optional features on (these features are off by default).
The extension works even if you browse BitcoinTalk.org without being logged in. The source code is not obfuscated and can be examined by anyone with sufficient JavaScript knowledge using developer tools built into most browsers. Same tools can also be used to check network traffic to/from the extension.