Goal: use logical volumes on a KVM host as virtual disks on KVM guests. The key in achieving the goal is to define a logical volume pool and then let libvirtd allocate virtual volumes for guests from that pool. The logical volume pool will be backed up by a LVM logical volume group.
Create a volume group for the logical volume pool. If you already have one, you can use it and skip this step.
vgcreate 'virtvg' '/dev/virtpv' # substitute /dev/virtpv with any block device suitable for your needs
Define a volume pool and make it start automatically when libvirtd starts so that it is always availabe for your guests.
virsh 'pool-define-as' \ --name 'virtvg' \ --type 'logical' \ --source-format 'lvm2' \ --target '/dev/virtvg' virsh 'pool-autostart' 'virtvg' virsh 'pool-start' 'virtvg'
Start the default network device:
virsh 'net-start' 'default'
Install a guest machine which uses a virtual volume from the newly created pool. If you would like to provide multiple volumes for your guest, just provide multiple --disk arguments. Note that the size of each volume must be explicitly given.
virt-install \ --hvm \ --name 'myvirtwheezy' \ --ram '1024' \ --disk 'pool=virtvg,bus=virtio,size=12' \ --network 'network=default,model=virtio' \ --os-type 'linux' \ --location 'http://ftp.fi.debian.org/debian/dists/wheezy/main/installer-amd64/'
From the guest's point of view, the logical volume (/dev/virtg/myvirtwheezy.img1) in host's virtvg volume group looks like an ordinary virtual volume (/dev/vda). The guest is free to use it as it sees appropriate, even managing it with LVM. Which would result in a nested LVM, extremly useful for creating runtime-resizable guests.