Indirectly kundalini can be awakened by devotion, by selfless service, or by intellectual enquiry. In these paths the blocks to the awakening of kundalini are slowly removed.
Occasionally, individuals on these paths will experience a sudden awakening of kundalini but generally because the blocks are slowly and gently removed kundalini-like experiences evolve slowly in these paths.
Broadly speaking there are two radically different direct approaches to awakening kundalini. One approach requires initiation by a guru and relies upon a technique called shaktipat, or "descent of shakti.'' It is variously called: Siddha Mahayoga, Kundalini Mahayoga or Sahaja Yoga (Spontaneous Yoga).
The other approach uses intentional yogic techniques . The styles using intentional techniques include Mantra Yoga, Hatha Yoga, Laya Yoga or Kriya Yoga. Fundamentally the approach of Siddha Mahayoga and the Kundalini Yogas are different. In Siddha Mahayoga the guru awakens the kundalini and after that the core of the practice is the inactive and non-willful surrender to kundalini.
In Kundalini Yogas the will is used to awaken the kundalini and to guide its progress. Clearly these are different approaches. Nevertheless, elements of the each approach occur in the practices of the other. Siddha Mahayogins may use asanas, pranayamas and other hatha yoga practices. On the other hand gurus in Kundalini Yoga may give infusions of shakti to their students to help them at particular points in their practice.
Since every practitioner brings his own unique inclinations and obstacles to the practice of yoga it is very hard to generalize on this point. In terms of actually awakening kundalini gurus of Siddha Mahayoga claim that the kundalini is more easily and reliably awakened by the grace of the guru than by individual effort. In my limited experience I would agree. with this assertion.
While not every long-term student of either practice necessarily shows signs of kundalini awakening it is amazing how many people have had instant awakenings of kundalini through initiation from siddha gurus.
In terms of encountering difficulties along the path the siddha gurus would also claim that fewer problems due to kundalini awakening, such as mental imbalance, are encountered by students of Siddha Mahayoga. Here I think the results are mixed. It seems to me that the guidance of the teacher in either Siddha Mahayoga or Kundalini Yoga is more a determining factor than which style of kundalini practice is employed.