/etc/xen/xend-config.sxpを編集してみる

 -(dom0-min-mem 256)
 +(dom0-min-mem 240)

あ、イミナシ。馬鹿杉。

libvir: Xen Daemon error : POST operation failed: 
(xend.err 'Error creating domain: I need 262144 KiB, 
but dom0_min_mem is 245760 and shrinking to 245760 KiB would leave only 260032 KiB free.')

(dom0-min-mem 232)にしてみて、再起動。