Virtio balloon driver

modulename: virtio_balloon.ko

configname: CONFIG_VIRTIO_BALLOON

Linux Kernel Configuration
└─>Device Drivers
└─>Virtio drivers
└─>Virtio balloon driver
In linux kernel since version 3.1 (release Date: 2011-10-24)  
This driver supports increasing and decreasing the amount
of memory within a KVM guest.

If unsure, say M.

source code:
drivers/virtio/virtio_balloon.c