I used to practice Aikido (for more than 10 years) and now teach Pekiti Tirsia Kali and have been doing so for a number of years.
Coming from an Aikido background to PTK, the difference between the two regarding knives is immense.
It'll be difficult to compare the two re knife defense but here goes:
Aikido in general trains to defend against a single slash or thrust, usually with the attacker taking a step forward. In PTK the maai is much closer - close enough to stab at arm's length, and the attack is never a single slash or thrust, rather a series of rapid and flowing cuts and stabs, with the empty hand assisting in the attack.
During my Aikido days, knife attacks were simple stabs and slashes, with the attacker allowing me to apply a technique. In PTK a knife attack is trained as a full on assault, with the attacker rushing in and using both the weapon and his empty hand to drive the knife to its targets. Blocking the knife attacks won't work, you'll just open up other targets for the attacker to get to as he redirects his weapon around your blocks. So applying a yokomen parry to apply a kotegaeshi will be answered by a quick slashing downward to the aikidoka's torso or thighs, followed by a thrust to the kidneys.
A properly trained PTK student will know all the usual empty handed knife defenses (blocks and locks like kotegaeshi and hiji kime) and will be trained to counter them. The way these are countered isn't't something I'd like to discuss in detail here but suffice to say that trying these defensive tactics on someone trained in PTK will get you killed much faster. Someone trained to use a knife might even want you to try grabbing for the weapon as bait for sneaky counterattack. When I was first started studying PTK I had the habit of trying to hold or grab the knife hand and I learned very quickly why I needed to change that habit LOL!
PTK empty handed knife defenses are rather hard to briefly describe however it is firmly rooted in the idea that the knife will be constantly and quickly moving and trying to chase after it is futile and suicidal. There is not a single magical technique but rather a series of defensive options based on a realistic understanding of what a knife can do and how it will be used. To learn to defend against a knife you need to know how it'll be used, which is why we usually teach knife use before knife defense. Part of the key to good knife defense is atemi, and lots of it.
I hope that I don't make PTK sound like a school of bloodthirsty assassins or I'm bashing aikido. I'm just giving my 2 cents on this, since PTK has been mentioned and I used to do aikido. I strongly recommend that aikidoka interested in investigating realistic knife defenses and attacks should invite a PTK teacher or student and see how the weapon is realistically applied.
Hope this helps..