CONFIG_XEN is not available for the selected architecture x86.
Result is shown for architecture arm

Xen guest support on ARM

configname: CONFIG_XEN

Linux Kernel Configuration
└─>Kernel Features
└─>Xen guest support on ARM
In linux kernel since version 2.6.23 (release Date: 2007-10-09)
Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.