On XenServer 6.5, I could find how to set the memory settings but I couldn't find how to read the current setting (what's the min, what's the max values) from the command line or by looking into a file.
Some posts on-line refers to /etc/default/grub file but that is for an older version of XenServer.
I just found my answer...
/boot/extlinux.conf
has the start up parameters.You can also run
/opt/xensource/libexec/xen-cmdline --get-xen dom0_mem
eg:
/opt/xensource/libexec/xen-cmdline --get-xen dom0_mem dom0_mem=4096M,max:4096M