Note: A VM image on NFS storage might not work. I had success with local and iSCSI storage.

• Create VM with virtio-scsi controler and unmap setting:

  --disk path=$IMAGEFILE,format=qcow2,bus=scsi,discard=unmap \
  --controller scsi,model=virtio-scsi \

• Note, use 'sda' in kickstart script, instead of 'vda'.
• Check if trim is supported:

  $ grep "" /sys/block/sda/queue/discard_*
  /sys/block/sda/queue/discard_granularity:4096
  /sys/block/sda/queue/discard_max_bytes:1073741824
  /sys/block/sda/queue/discard_zeroes_data:0
  
  $ cat /sys/block/sda/device/scsi_disk/*/provisioning_mode
  unmap
  

• Add discard option to /etc/fstab, or schedule fstrim operation.
• Configure /etc/lvm/lvm.conf:

  issue_discards = 1

• reboot

• Monitor VM image size. Use the -s ls option to see real usage.
• Create large file:

  cd /var/tmp/
  time dd if=/dev/urandom of=tstfile bs=2048k count=512 oflag=direct

• Delete tstfile
• Trim the filesystem:

  fstrim -v /var

• Check VM image size. You can see in the first column the size grow and shrink after fstrim.

  $ ls -ls vmcentos.qcow2
  1226824 -rw-r--r-- 1 nobody kvm 1256325120 Dec  2 21:59 vmcentos.qcow2
  1355912 -rw-r--r-- 1 nobody kvm 1388511232 Dec  2 21:59 vmcentos.qcow2
  1552968 -rw-r--r-- 1 nobody kvm 1589837824 Dec  2 21:59 vmcentos.qcow2
  1755128 -rw-r--r-- 1 nobody kvm 1796943872 Dec  2 22:00 vmcentos.qcow2
  1949832 -rw-r--r-- 1 nobody kvm 1996685312 Dec  2 22:00 vmcentos.qcow2
  2156152 -rw-r--r-- 1 nobody kvm 2207477760 Dec  2 22:00 vmcentos.qcow2
  2245256 -rw-r--r-- 1 nobody kvm 2299199488 Dec  2 22:00 vmcentos.qcow2
  2245256 -rw-r--r-- 1 nobody kvm 2299199488 Dec  2 22:00 vmcentos.qcow2
  2245256 -rw-r--r-- 1 nobody kvm 2299199488 Dec  2 22:00 vmcentos.qcow2
  2245256 -rw-r--r-- 1 nobody kvm 2299199488 Dec  2 22:00 vmcentos.qcow2
  2245256 -rw-r--r-- 1 nobody kvm 2299199488 Dec  2 22:00 vmcentos.qcow2
  1130376 -rw-r--r-- 1 nobody kvm 2299199488 Dec  2 22:00 vmcentos.qcow2
  1130376 -rw-r--r-- 1 nobody kvm 2299199488 Dec  2 22:00 vmcentos.qcow2
  1130376 -rw-r--r-- 1 nobody kvm 2299199488 Dec  2 22:00 vmcentos.qcow2
  1130376 -rw-r--r-- 1 nobody kvm 2299199488 Dec  2 22:00 vmcentos.qcow2