KVM and OpenVZ are probably the most popular terms if you are doing research about VPS. To explain difference between both, here we have an article which explains difference between KVM and OpenVZ visualizations
OpenVZ is a very commonly available type of VPS technology. When compared with KVM, OpenVZ is cheap and light weight. Some say that due to this light weight nature, OpenVZ is faster than KVM. Unlike KVM, OpenVZ can only run Linux OS and can not run any other OS like windows and some other custom OS types.
Although being fast and cheap, OpenVZ has some drawbacks too, some web hosts tend to oversell their OpenVZ servers. And OpenVZ has a shared kernel which means we can not customize kernel.
KVM is one of the most popular types of VPS nowadays. It offers better scalability and virtualization as it is a hardware virtualization. Which means a user can install any type of OS on a KVM VPS. KVM is also more isolated than the OpenVZ and gets 100% of resources allocated to the user. While KVM looks better, it has some drawbacks too, it is much more expensive than OpenVZ and might appear complex for some users especially if they are not experienced.
Which one should I Choose?
I would say that You should go with KVM and it is my opinion. It provides better isolation and allows you to use any OS of your choice. It provides better performance as a bonus. OpenVZ, on the other hand, is simple and easy to use. If you are someone who has no experience with VPS, OpenVZ would be a better option for you. Also, if you are on a tight budget and wants to save some bucks, you can consider using OpenVZ. To conclude, I would say that both has its own merits and demerits you should let your discretion to choose one. You should choose it as per your need. As I said earlier, I would go with a KVM machine as it suits my needs perfectly