Finally I found the configuration that works for me :
TIMEOUT 30
DEFAULT primary
MENU TITLE L4T boot options
#LABEL primary
#MENU LABEL primary kernel
#LINUX /boot/Image
#INITRD /boot/initrd
#APPEND ${cbootargs}
LABEL primary
MENU LABEL primary kernel
LINUX /boot/Image_kvm
FDT /boot/tegra210-p3448-0000-p3449-0000-a02-kvm.dtb
INITRD /boot/initrd
APPEND ${cbootargs}
basically the file which works for me is for the a02 version. but my nano is b01. why ?
root@zioma-desktop:/home/zioma# kvm-ok
INFO: /dev/kvm exists
KVM acceleration can be used
PS : it worked one time only.