Hello Jan,
A very nice answer indeed! Allow me to correct one slight inaccuracy, though.
In addition - PVST+ supports both ISL and 802.1q trunking. That's why it's called Per-VLAN spanning tree plus anyway.
There actually is a difference between PVST and PVST+. The PVST is historically older, and was the mode of running several STP instances - one for each VLAN - over an ISL trunk in particular. Because ISL encapsulates the entire original frame and has no concept of native VLAN, PVST merely dealt with encapsulating individual BPDUs using ordinary ISL framing, and setting the 'BPDU' flag in the ISL header to signify that this frame is to be processed by the CPU of the receiving switch.
PVST+ came later with running several STP instances over 802.1Q trunk in particular. Things are more complicated here. First, because there is a concept of a native VLAN on 802.1Q trunks, you can also have a native VLAN mismatch, which could, among other things, cause the STP in the two mismatched VLANs to misbehave. Therefore, PVST+ has to somehow be able to detect native VLAN mismatches and prevent them from wreaking havoc. This is performed by adding a special TLV record at the very end of each PVST+ BPDU that contains the VLAN ID where this BPDU was first originated. If this BPDU is received on a different VLAN (thanks to mismatched, removed or rewritten 802.1Q tags) than the VLAN recorded in this TLV, the receiving switch will declare a PVID Inconsistency and block the port, thereby preventing possible switching loops. See the following document for details:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00801d11a0.shtml
Second, the switch behind the 802.1Q trunk may not be aware of per-VLAN STP but it still may be running a single instance of IEEE-compliant STP, creating a so-called Common Spanning Tree, i.e. a single instance of ordinary STP which enforces a common shared spanning tree for all VLANs outside. The PVST+ has to somehow work together with the pure STP running outside. The PVST+ does this by always sending ordinary STP BPDUs for VLAN 1 untagged (regardlessly of the native VLAN on the trunk), and sending PVST+ BPDUs for all VLANs accordingly tagged and SNAP-encapsulated, thereby tunneling them through the CST cloud.
So the PVST was the former way of running several STP instances over an ISL trunk (note no interworking or consistency checking was necessary here: the only vendor that spoke ISL was Cisco), PVST+ is the method of running several STP instances over an 802.1Q trunk including consistency checks and possible interworking with plain STP.
Best regards,
Peter
