The Xen balloon driver supports two operations:
 1. Inflating - which means domain giving up pages of memory to Xen.
 2. Deflating - which means reclaiming memory pages from Xen.

Currently a 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.