LVM is extremely faster and reliable especially than a (cheap) network shared storage system. To be safe and fast using SAN are needed, at least, two different syncronized storage units, two different ethernet infrastructure (2 switch, 2 ups, 802.3ad, etc.), 10Gbps speed connection and a commercial storage software (like open-e, drbd sucks).
LVM is also better than local storage for the snapshot feature, allowing backups of running kvm systems.
The only big missing thing is an easy live migration feature between different hosts in a cluster. That's why my question...