On newer distros normally udev will create these devices but in some cases it doesnt know how to. Our distros are deployed with the devices already created so that it's not a problem. If you're building your own distro or managed to fubar your /dev directory however, you may need to create the xvd[a-h] block devices manually. The mknod commands below will (re)create these devices.
mknod /dev/xvda b 202 0 mknod /dev/xvdb b 202 16 mknod /dev/xvdc b 202 32 mknod /dev/xvdd b 202 48 mknod /dev/xvde b 202 64 mknod /dev/xvdf b 202 80 mknod /dev/xvdg b 202 96 mknod /dev/xvdh b 202 112