got solution ...
when i just peeped into my friends systems, which has RED HAT 9 and FEDORA2. Both OS have /dev/fb, but when i installed Xfbdev on them, same problem came.
then i googled for it, and found some important things that you have to specify the framebuffer size and other parameters in boot options, then only device for framebuffer will be created (in case of udev - fedora 3 and 4). i have added following lines in my grub.conf and installed it on mbr. and everything worked.
title Fedora Core (2.6.11-1.1369_FC4) Xfbdev & framebuffer
kernel /vmlinuz-2.6.11-1.1369_FC4 ro root=LABEL=/1 rhgb quiet vesafb:mtrr,ywrap vga=0x318
here i just added "vesafb:mtrr,ywrap vga=0x318", other things were present previously.
and my xfbdev server is running well.
thanx for everyone who showed interest in solving my problem.