Difference between different unix versions

[NJITX::HXN8477]

I know this is a stupid question, but I'll ask it any way because I am
VERY new to unix.  As far as I know, there are 3 flavors of unix: BSD 
which is public domain, System V which is moderately priced and Xenix
which is astronomically expensive.  Just what is the difference between
the 3 flavors.  And if all of them can run any unix program, what makes
one buy Xenix, when they can get BSD?  

                    
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|Hamed Nassar               |Internet  : hxn8477%njitx.decnet at njitc.njit.edu |
|EE Department              |UUCP      : bellcore!argus!mars!nancy           |
|NJ Institute of Technology |CompuServe: 74000,130                           |
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