OpenVZ is least expensive for the hoster (i.e., you) if you only offer linux hosting - less problems, better speed, better resource sharing, etc. OpenVZ tends to be massively oversold on hosts - that gives bad impression of it to most people. If you do not oversell, OpenVZ is faster that Xen. You *will* need slight memory adjustments for OpenVZ machines due to its unconventional memory accounting, which leads to VMs *reporting* (not actually using, but reporting) higher memory usage that Xen counterparts, but that's another discussion.
For Xen, however, there is a possibility to run any OS, not just Linux - that's why many hosters provide both OpenVZ and Xen. I would advise against Xen at all and suggest using KVM if you need other OS, but this discussion is well outside the scope of this topic.
Yes, i want sell vps.
i don't have target at this subject, just offer BEST SERVICE !
I think you need a bit more than that to stay competitive in this industry.
From a consumer's point of view, unless there's a need for custom kernels and running java, OpenVZ is a great choice. XEN tends to sell better due to the belief that it cannot be oversold, however, it can.
Ultimately, it comes down to the provider and how well service and performance is, relative to price. As HUGOBLUE mentioned, it helps targeting a niche or a unique way of pitching yourself and your business.