after you create the pv
you do this: vgdisplay testvg | grep "Total PE"
lvcreate -l 10230 testvg -n "whatevername"
not 10230
-l is the number you got from the grep
then it will be in /dev

  vgcreate backup_sdb /dev/sdb
  vgdisplay backup_sdb | grep "Total PE"
  lvcreate -l 131274 backup_sdb -n "lv_sdb"
  pvdisplay /dev/sdb
  lvcreate -L500000 -nVdomains backup_sdb
  pvdisplay /dev/sdb
  mount /dev/backup_sdb/Vdomains /mnt/test
  mke2fs -j /dev/backup_sdb/Vdomains
  lvcreate -L100000 -nserver3701 backup_sdb
  mke2fs -j /dev/backup_sdb/server3701
  mount
  dmesg