Please try to chmod the rtl_sdr usb device /dev/xxx (i can't remember ..) then start gqrx as user. Check if it segfaults due a missing library.... Or try the older gqrx. Talking about dvb, we don't have the dvb driver for the rtl_sdr devices into kernel power. If we had this kind of module, we should unload it before use rtl_fm.