I'm going to skip the lecture about how stupid it was to install a beta OS with no backup and tell you that yes, you may be able to go back to Mavericks through the Recovery partition, but frankly it will be something of a miracle if your data survives this process entirely intact.
And of course, if the reason you can't boot into the OS anymore is because the boot partition is damaged, there's no guarantee that going back will successfully boot either.
There's no sure way to fix this problem without at least a RISK of losing your files, but if you have any friends who also have Macs you might be able to get one of them to mount your computer as though it were an external hard drive through Firewire Target Disk Mode, but both computers must have Firewire for this to work.
If you can do that, you should be able to salvage your personal files. But that's a lot of ifs.