Virtual devices are named internally in Xen using the Linux-style major and minor numbers. This is true even for non-Linux virtual machines. The number returned from xm block-list is a decimal representation of the combined major and minor numbers.
For example, if a virtual machine’s virtual disk is designated as hda, the number returned from xm block-list is 768. hda has major number 3 and minor number 0. The major number is stored as a high-order byte; the minor is the lower byte. A decimal representation is (3*256)+0 = 768. Another example, sda3 has major number 8 and minor number 3, so its decimal representation is (8*256)+3 = 2051.
Of course, it is possible to manually work backwards from a number to discover the human-readable device name. For your reference, some common mappings listed in the following table.