How do they do their VPS product? [was Re: FreeBSD based web hosting?]

[George Hartzell]

Sahil Tandon writes:
 > Maxim Khitrov <mkhitrov at gmail.com> wrote:
 > 
 > > I'm currently with JohnCompanies. Overall, it's been a positive
 > > experience, though I wish they offered FreeBSD 7. Beta testing for it
 > > was supposed to begin last month, but so far no news.
 > 
 > +1 for JC.  The tech support and availability is unparalleled.  Satisfied 
 > customer for over three years.

Seeing JC advertise a FreeBSD VPS product reminded me of an itch I've
never been able to scratch.

I know about jails at the simple end, and I know about VMware at the
complicated end, but I've never been able to figure out what technology
underlies the VPS offerings.

I help out a friend on a Verio VPS that reports itself as FreeBSD 4.7
w/ a kernel config file named VKERN.  There's some concept of a
virtual root, a command named 'virtual' works a bit like sudo does for
credentials, and there's otherwise an odd mix of "it's all my machine"
and "it's a shared server".

Is/was there some proprietary virtualization technology in the 4.x
days?  Is it still alive and kicking somewhere?

Anyone have any insights into how these VPS products were built?

Thanks,

g.