The KVM and the CVM are targeted at two different types of consumer devices. The former runs on small devices with small memory footprint and computation power (cell phones) while the latter is used on more powerful devices with more memory and computation power (PDA,...)
Here is what Sun says about them:
KVM:
...Sun's K virtual machine (KVM) is a virtual machine designed from the ground up with the constraints of inexpensive mobile devices in mind. Named to reflect that its size is measured in the tens of kilobytes, CLDC is suitable for devices with 16/32-bit RISC/CISC microprocessors/controllers, and with as little as 160 KB of total memory available -- 128 KB of which is for the storage of the actual virtual machine and libraries themselves.
CVM:
Typically, these devices run a 32-bit microprocessor/controller and have more than 2.0MB of total memory for the storage of the virtual machine and libraries. CDC contains the CVM virtual machine. CVM is a full-featured virtual machine designed for devices needing the functionality of the Java 2 virtual machine feature set, but with a smaller footprint.
[ February 26, 2002: Message edited by: Valentin Crettaz ]