Spoiler:
Kylo, for reasons unknown to him, felt his powers weaken upon killing his father:
"Stunned by his own action, Kylo Ren fell to his knees. Following through on the act ought to have made him stronger, a part of him believed. Instead, he found himself weakened."
--The Force Awakens
More info on the impact Kylo's wound had on him:
"Kylo Ren struggled to stand."
--The Force Awakens
Kylo, at least in the novel, was only toying with Finn until he was tagged, after which he immediately ended the fight:
"Taking a step back, Ren reconsidered his opponent. When he closed the distance between them, it was with a purpose that had been previously lacking. Expecting an execution, he had found a contest. Now he had been touched. It was time for play to end. Advancing relentlessly, he was driven by something Finn could not even sense, far less counter. Still the ex-troper fought back, until Ren landed a blow that cut across Finn's chest and sent the lightsaber flying from his hand. It landed in the snow six meters distant. It was over."
--The Force Awakens
The clash between Rey and Kylo seems very reminiscent in principle to Anakin vs Dooku:
"Expecting weakness, he encountered only strength. Her skill with the device was raw at best, but it was backed by a fury that was as new to his experience as it was unexpected.
[...]
To an observer in the distance, it would have appeared as if a series of small explosions were going off in the depths of the forest. Blow after blow landed as lightsaber struck against lightsaber. Though Ren was bigger and stronger than Rey, their struggle had nothing to do with physical size. What she lacked in mass, she made up for in ferocity.
[...]
A long moment passed, in which Ren sensed a change in the air, a change in HER. Then she opened her eyes and attacked, viciously, in a way she didn't know she was capable of, striking again and again as Ren was slowly driven back."
--The Force Awakens
Ren seemingly could have killed Rey twice during in the fight, and hesitated due to confusion when she closed her eyes:
"Ren held his lightsaber, poised to strike. "I could kill you right now. But there is another way."
[...]
Rey closed her eyes. Ren hesitated, confused by her actions."
--The Force Awakens
Also, just a cool side-note, Ren is apparently partially able to hold her off even after being disarmed, at least in the novel:
"Unarmed, he raised a hand and utilized the Force to fend off one slashing blow after another, until finally her fury penetrated his remaining defenses."
--The Force Awakens
Obviously this doesn't change everything, but it's a little "good to know".