In simple terms, the difference between Virtuozzo and Xen is that the former is proprietary and must be licensed per VM. Xen is open-source and is free to use by anybody. OpenVZ is the free version of virtuozzo but it is limited to the command line.
RBL stands for Realtime Black lists and it mainly works for email servers. RBL is a collections of IP addresses or network addresses which has been found to be spamming. So the email servers will connect to these RBLs before accepting email from an IP and make sure that the IP from with the mail arrives is free from spams.
Change default FTP port for greater security for the server and to its contents, if the server is a shared one. Not only because the default port gives you a connection automatically and if they could guess the password, your content is in risk. So, I would suggesto to change the FTP port to a non standard one.
Nearly all abusive e-mail messages carry fake sender addresses. To help prevent this sort of abuse, it is recommended to add SPF record as well as DKIM to publish yourself and your identity.