You have to install the lvm2 driver in Ubuntu and then mount Fedora, so grub2 can find it. Then it works.

There is little advantage to LVM if really using it as one partition amount many. It is intended as an entire drive (or more) to make it easy to resize partitions. But it adds a level of complication. Many here multi-booting just install Fedora to a partition.

Advantages/Disadvantages LVM Post #9
http://ubuntuforums.org/showthread.php?t=1586328
lvm info older:
http://ubuntuforums.org/showthread.php?t=141900
http://tldp.org/HOWTO/LVM-HOWTO/index.html