Solution:

A) Host operating system is Asianux Server 3 SP1:
when installing Red Hat Enterprise Linux 5.1 as a guest OS, please choose English when installing.

B) host operating system is Red Hat Enterprise Linux 5.1
Please use the ftp protocol as the other protocols combined with paravirtualisation will fail.

Host OS  Guest OS    Tool               Protocol      Results
AX30sp1  rhel5.1       virt-manager   para+nfs     ok (install guest os in English)
AX30sp1  rhel5.1       virt-manager   para+ftp     ok (install guest os in English)
AX30sp1  rhel5.1       virt-manager   para+http   ok (install guest os in English)
rhel5.1    AX30sp1b5  virt-manager   para+nfs    fail
rhel5.1    AX30sp1b5  virt-manager   para+ftp    ok
rhel5.1    AX30sp1b5  virt-manager   para+http  fail