Yesterday I've decided to give centos xen a try and I get this messages also.

[root@fhpxen1 ~]# dmesg  -c
System RAM resource 48000000 - 4fffffff cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17
device vif10.0 entered promiscuous mode
ADDRCONF(NETDEV_UP): vif10.0: link is not ready
device vif10.0-emu entered promiscuous mode
virbr0: topology change detected, propagating
virbr0: port 2(vif10.0-emu) entered forwarding state
virbr0: port 2(vif10.0-emu) entered forwarding state
System RAM resource 48000000 - 4fffffff cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17
System RAM resource 48000000 - 4fffffff cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17
System RAM resource 48000000 - 4fffffff cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17
System RAM resource 48000000 - 4fffffff cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17
System RAM resource 48000000 - 4fffffff cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17
System RAM resource 48000000 - 4fffffff cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17
System RAM resource 48000000 - 4fffffff cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17
System RAM resource 48000000 - 4fffffff cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17
System RAM resource 48000000 - 4fffffff cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17
System RAM resource 48000000 - 4fffffff cannot be added
xen_balloon: reserve_additional_memory: add_memory() failed: -17
System RAM resource 48000000 - 4fffffff cannot be added


[root@fhpxen1 ~]# cat /sys/devices/system/xen_memory/xen_memory0/info/current_kb
1048568
[root@fhpxen1 ~]# cat   /sys/devices/system/xen_memory/xen_memory0/target_kb
1048576
[root@fhpxen1 ~]#

[root@fhpxen1 ~]# xl info
host                   : fhpxen1
release                : 3.4.54-8.el6.centos.alt.x86_64
version                : #1 SMP Tue Jul 23 17:29:30 UTC 2013
machine                : x86_64
nr_cpus                : 8
max_cpu_id             : 7
nr_nodes               : 1
cores_per_socket       : 4
threads_per_core       : 2
cpu_mhz                : 3400
hw_caps                : bfebfbff:2c100800:00000000:00007f40:77fafbff:00000000:00000021:00002fbb
virt_caps              : hvm hvm_directio
total_memory           : 32630
free_memory            : 28071
sharing_freed_memory   : 0
sharing_used_memory    : 0
free_cpus              : 0
xen_major              : 4
xen_minor              : 2
xen_extra              : .2-23.el6
xen_caps               : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 hvm-3.0-x86_32p hvm-3.0-x86_64
xen_scheduler          : credit
xen_pagesize           : 4096
platform_params        : virt_start=0xffff800000000000
xen_changeset          : unavailable
xen_commandline        : dom0_mem=1024M,max:1024M loglvl=all guest_loglvl=all
cc_compiler            : gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-3)
cc_compile_by          : mockbuild
cc_compile_domain      : centos.org
cc_compile_date        : Thu Jul 18 13:42:23 UTC 2013
xend_config_format     : 4