XEmacs was a branch version of Emacs that is no longer actively developed. Originally known as “Lucid Emacs”, XEmacs was forked from a prerelease version of Emacs 19. XEmacs last released a new version on January 30, 2009, which lacks many important features that exist in Emacs. Since its development has stopped, we do not expect to see any new releases.
In the past, it was not uncommon for Emacs packages to include code for compatibility with XEmacs. Nowadays, most built-in and third party packages have either stopped supporting XEmacs or were developed exclusively for Emacs.
If you want to talk about these two versions and distinguish them, please call them “Emacs” and “XEmacs.” To contrast “XEmacs” with “GNU Emacs” would be misleading, since XEmacs too has its origin in the work of the GNU Project. Terms such as “Emacsen” and “(X)Emacs” are not wrong, but they are not very clear, so it is better to write “Emacs and XEmacs.”