So in summary (some assumptions):

1) You have Xen installed and some distro of linux running as a VM
2) You have xinetd installed and running
3) You installed Firebird classic
4) You restarted xinetd and see that port 3050 is listening (netstat -an
| grep 3050)

You should really try to use isql now (/opt/firebird/bin) to try to
connect to a sample fdb on the server and see what (if any) error
messages you get now. Of course if my summary of where you are not is
wrong then this will not help...

-steve