There can be a number of scenarios you want to extend an LVM.
a. New Hard Drive
b. Free Space
c. Extended disk in a VM
If you just extended space, you have that as raw filesystem. So just open fdisk and create new partition and start working on the lvm:
vgextend "lvmname" /dev/sda<x>
Find out the size of the extended Volume :
From the information that you see as a result of above commands output, add the Free extends and Number of PE which will give the number of new extend.
Now run :
`lvm lvresize -l
lvm lvresize -l 4900 /dev/VolGroup/lv_root