There's 73GB to use. Enough for a 10GB Puppetmaster and a couple of 5GB clients.
apprentice@xenserver:~$ sudo fdisk /dev/sdb
<snip>
Command (m for help): n
Partition type:
p primary (0 primary, 0 extended, 4 free)
e extended
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-143374737, default 2048):
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-143374737, default 143374737):
Using default value 143374737
Command (m for help): t
Selected partition 1
Hex code (type L to list codes): 8e
Changed system type of partition 1 to 8e (Linux LVM)
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
apprentice@xenserver:~$ sudo pvcreate /dev/sdb1
Writing physical volume data to disk "/dev/sdb1"
Physical volume "/dev/sdb1" successfully created
apprentice@xenserver:~$ sudo vgcreate vgvhosts /dev/sdb1
Volume group "vgvhosts" successfully created
Don't. When creating xen VMs, the command xen-create-image will create LVs as well.