"When i boot my PC with XEN kernel, wvdialconf refuses to detect serial ports, in turn my Modem connected to COM1. But it works properly when i boot my PC without XEN kernel."
I don't think your problem is with how wvdial is configured. I think that your problem is with how COM1 is configured in Xen. Here is an explanation of how to configure COM1 in Xen when Xen does not do so automagically:
In order to configure Xen serial console output, it is necessary to add a boot option to your GRUB config; e.g. replace the previous example kernel line with:
kernel /boot/xen.gz dom0_mem=131072 com1=115200,8n1
This configures Xen to output on COM1 at 115,200 baud, 8 data bits, 1 stop bit and no parity. Modify these parameters for your environment.