Xeno Balloon driver supports two operations:

1. Inflating - which means domain giving up pages of mem to xen.
2. Deflating - which means reclaiming memory pages from xen.

Currently, domain can only claim pages from xen up to the number of 
previously released ones. This is to change. 

Example:

# balloon inflate 1000

Give up 1000 pages to xen.

# balloon deflate 1000

Claim 1000 pages from xen.