CONFIG_XEN_STUB is not available for thedefaultarchitecture x86.
Result is shown for architecture 64BIT

Xen stub drivers

modulename: xen-stub.ko

configname: CONFIG_XEN_STUB

Linux Kernel Configuration
└─>Device Drivers
└─>Xen driver support
└─>Xen stub drivers
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Allow kernel to install stub drivers, to reserve space for Xen drivers,
i.e. memory hotplug and cpu hotplug, and to block native drivers loaded,
so that real Xen drivers can be modular.

To enable Xen features like cpu and memory hotplug, select Y here.

source code:
depends
CONFIG_BROKEN
CONFIG_X86_64
CONFIG_XEN