Krav maga is a self-defense oriented martial art. It was developed as a means of self-defense for those Europeans concerned with anti-Semitic extremism. It is currently used by the Israel Defense Forces.
The underlying goal of Aikido is to protect your self while mitigating injury to your assailant.
Krav Maga is intended for self-protection and isn't sport oriented. The overall strategy is to prevent injury, practice dignity while defending yourself. Their belief is to do whatever is needed to cause as much damage as possible to your attacker and get away safely. It is simple, ruthless and efficient. Krav Maga looks primal in nature and aggressive.
Both disciplines have a self-defense aspect to them. Gracie Jiu-Jitsu or GJJ places a heavier emphasis on self-defense than more sport oriented Brazilian Jiu-Jitsu. The self-defense portion utilizes ground fighting techniques but also incorporates weapons training, closing distance to clinch, takedowns and powerful strikes.
However, this is where the similarities end.
Krav Maga is an eclectic system that takes useful elements from existing martial arts. Most notably it incorporates:
Strikes - similar to Karate and Boxing
Take-downs / Throws - like Judo, Aikido, and Wrestling
Ground Fighting - similar to Judo and Wrestling
Along with this, Jiu Jitsu usually incorporates a gi while Krav usually doesn't have a unified uniform.
While Jiu-Jitsu and other martial arts may have certain etiquette in terms of their strikes Krav Maga focuses on self-preservation. So, they permit and train in groin strikes, eye pokes etc. The style is centered around survival and their techniques are a means to an end.
Many Krav Maga organizations do not support a competition component. Their belief is that Krav Maga is not a sport. They maintain a belief that sports operate under principles of using safe techniques, doing minimal harm and wearing down opponents using tactics supported by the "rules" of safe competition.
Their belief is that Krav Maga techniques do maximum harm and significant damage and therefore is not suitable for competition purposes. They may have never seen a high-level MMA fight.
While Krav Maga works in self-defense situations against Jiu-Jitsu the Krav Maga specialist would be like a fish out of water.
The Jiu-Jitsu fighter would close distance to clinch or initiate a takedown and establish side control or mount. A high-level BJJ practitioner would soften the Krav Maga fighter up with strikes from the top position until they exposed themselves and set in a quick submission.